Study Notes Exam 3
Evolution of Animals
- Animal History
o the fossil record is more complete for hard-shelled animals; possible evolutionary relationships are primarily worked out by using anatomical criteria
o there are over 30 animal phyla
- Criteria for Classification
o Levels of Organization: animals may have the cellular, tissue, or organ level of organization
§ germ layers refer to the number of layers of tissues
§ most animals are made of 3 tissue layers: endoderm, ectoderm and mesoderm
§ animals with only ectoderm and endoderm are limited to a tissue level of organization
§ animals with three tissue layers develop an organ level of organization
o Type of Body Plan
§ animals with a sac plan have an incomplete digestive system with only one opening used for both entrance and exit of food
§ animals with the tube-within-a-tube plan have a complete digestive system with separate entrance and exit to the digestive system; this allows specialization zones along the digestive tract
o Types of Symmetry
§ asymmetry: there is no particular symmetry
§ radial symmetry: body parts arranged around an axis, like spokes of a wheel
· radially symmetrical animals may be sessile (i.e., attached to a substrate or less motile)
§ bilateral symmetry: a body having a right and left, or complementary halves (one longitudinal cut down the center produces mirror halves)
· bilaterally symmetrical animals tend to be active and move forward
· cephalization: head development to localize the brain and sensory organs
o Type of Coelom
§ true coelomates possess a body cavity or coelom completely lined with mesoderm
§ acoelomates lack a body cavity or coelom although they have mesoderm
§ pseudocoelomates possess a pseudocoelom; the body cavity is incompletely lined by mesoderm; the cavity develops between the mesoderm and endoderm
§ coelomates are either protostomes or deuterostomes
§ protostomes develop with the first embryonic opening becoming the mouth
§ deuterostomes develop with the second opening becoming the mouth
o Segmentation is repetition of body parts along the length of the body
§ among coelomates, molluscs and echinoderms are non-segmented
§ annelids, arthropods, and chordates are segmented
§ segmentation leads to specialization of parts as they differentiate for specific purposes
Multicellularity
- Phylum Porifera (~ 5,000 species of sponges)
o sponges have no symmetry and no tissues (unique in the animal world)
o sponges remain at a cellular level of organization, having evolved separately from protozoan ancestors
o sponges are aquatic, largely marine animals, that vary in size, shape, and color
o their saclike bodies are perforated by many pores; Porifera means ³pore-bearing
o several types of cells are found in a sponge
§ the outer layer of their wall contains flattened epidermal cells; some have contractile fibers
§ middle layer is a semifluid matrix with wandering amoeboid cells & spicules
§ inner layer is composed of collar cells (choanocytes)
§ there are no nerve cells or means of coordination between cells
o beating collar cells produce currents through pores in wall into a central cavity and out through osculum
o simple sponges 10 cm tall can filter as much as 100 liters of water a day
o sponges are sessile (donıt move) and filter feeders
§ collar cells engulf and digest food particles in food vacuoles, or pass them to amoeboid cells
§ amoeboid cells transport nutrients from cell to cell, and they also produce gametes and spicules
o sponges reproduce asexually by budding, which can produce quite large colonies
o fragmentation occurs when sponges are chopped up; cells reorganize & each piece can start a complete sponge
o sponges reproduce sexually when eggs and sperm are released into a central cavity; the zygote develops into a ciliated larva
o sponge classification is partly based on its skeleton composed of spicules
§ spicules are tiny needle-shaped structures with one to six rays
§ chalk sponges have spicules composed of calcium carbonate
§ glass sponges have spicules of silica
o spongin is a protein made of modified collagen, it gives a sponge its flexibility
Two Tissue Layers
- Phylum Ctenophora (~ 90 species of comb jellies)
o comb jellies develop only ectoderm and endoderm (are diploblasts), and are radially symmetrical
o comb jellies are transparent and often luminescent; their eight plates of fused cilia resemble long combs
o most of their body is a jellylike packing material called mesoglea
o they are the largest animals to be propelled by beating cilia
o long tentacles covered with sticky filaments - or an entire body covered by sticky mucus - captures prey
- Phylum Cnidaria (~ 9,000 species of cnidarians)
o cnidaria also develop only ectoderm and endoderm and are tubular or bell-shaped
o they mostly live in coastal waters but oceanic jellyfish and freshwater hydra exist
o cnidaria have cnidocytes, a specialized cell that contains a nematocyst
§ the nematocyst is a fluid-filled capsule, with a long coiled hollow thread
§ when the trigger of the cnidocyte is touched, the nematocyst is discharged
§ some threads merely trap prey or predators; others have spines that penetrate and inject paralyzing toxins
o a cnidarian body is a two-layered sac with the epidermis derived from ectoderm & the inner tissue layer derived from the endoderm
o the inner gastrovascular cavity digests food and circulates nutrients
o there are muscle fibers at the base of the epidermal and gastrodermal cells
o nerve cells located below the epidermis near the mesoglea interconnect and form a nerve net
§ unlike highly organized nervous systems, this nerve net transmits impulses in several directions at once, which results in multiple firings of nematocysts in parts not directly stimulated
§ having both muscle fibers and nerve fibers, these animals are capable of directional movement, the body can contract or extend, and tentacles that ring the mouth can extend to grasp prey
o cnidaria have two basic body plans
§ a polyp is vase-shaped and the mouth is directed upward
§ a medusa is bell-shaped and the mouth is directed downward
§ when both stages are present, the animal is dimorphic:
· the polyp stage is sessile and produces the medusae
· the medusa stage is motile and produces the egg and sperm
- Cnidarian Diversity
o Class Anthozoa: sea anemones and corals
§ sea anemones are solitary polyps that attach to rocks, timbers, etc., or may be mutualistic and attach to a hermit crabıs shell
§ many are brightly colored and resemble flowers
§ a thick, heavy body supports a mouth that is surrounded by hollow tentacles; they feed on invertebrates and fish
§ corals may be solitary; most are colonial
§ most corals live in shallow waters; the accumulation of their calcium-carbonate remains builds reefs
o Class Hydrozoa includes hydras and Portuguese man-of-war (Physalia)
§ the Portuguese man-of-war is a colony of polyps; the original polyp becomes a gas-filled float
§ other polyps bud to specialize for feeding or reproduction
§ tentacles having numerous nematocysts can cause serious injury to swimmers
o Class Scyphozoa includes the true jellyfishes (e.g., Aurelia)
§ the medusa stage is dominant in jellyfish; the polyp remains small
§ jellyfishes are part of the zooplankton, the food for larger marine animals
- Hydra and Obelia
o hydra are solitary, freshwater hydrozoan polyps with 4-6 tentacles containing nematocysts surrounding the mouth
o hydras move by gliding or somersaulting
o hydras have both muscular and nerve fibers, and respond to touch
o Hydra anatomy
§ epidermal cells are termed epitheliomuscular cells and contain muscle fibers
§ cnidocytes and sensory cells are in the epidermis
§ interstitial cells can produce an ovary or testis, and may assist regeneration
§ gland cells secrete digestive juices into a gastrovascular cavity; the tentacles capture and stuff in prey
§ digestion is completed within food vacuoles of nutritive-muscular cells; nutrients then diffuse to the rest of the cells in the body
o Hydra reproduction
§ hydra reproduces asexually by budding
§ in sexual reproduction, sperm from a testis swim to an egg within an ovary; after early development within an ovary, a protective shell allows the egg to survive until conditions are optimum for it to emerge
o Obelia is a colony of polyps enclosed in a hard, chitinous covering
§ feeding polyps have nematocysts and extend beyond the covering to capture tiny worms, etc.; connected polyps distribute food through colony
§ a colony increases in size by asexually budding new polyps
§ reproductive polyps bud off the medusae stage which is free-living or attached to the colony
§ Obelia medusae tentacles have nematocysts; the gastrovascular cavity extends into tentacles
§ a nerve net runs in two rings around the bell margin; it has statocysts for balance and ocelli to detect light
§ sperm and egg from medusae unite; the resulting zygote develops into a ciliated planula larva that settles down to develop into a polyp colony
Bilateral Symmetry
- Triploblasts: all animals beyond this point are triploblasts with three germ layers
- flatworms have a sac body plan; ribbon worms have a tube-within-a-tube body plan
- the tube-within-a-tube plan allows specialization of parts along the tube
- Phylum Nemertea (~ 650 species of marine ribbon worms)
o ribbon worms have a distinctive eversible proboscis stored in a rhynchocoel
o the proboscis is a long, hollow tube that is shot outward through a pore located above the mouth; it is used for prey capture, defense, locomotion, and burrowing
- Flatworms: Phylum Platyhelminthes (13,000 species of flatworms)
o planaria and relatives are freshwater animals in the class Turbellaria
o flukes are external or internal parasites in the class Trematoda
o tapeworms are internal parasites in the class Cestoda
o in addition to endoderm and ectoderm, a mesoderm layer gives rise to muscles and reproductive organs
o there is no coelom; they are acoelomates
o a branched gastrovascular cavity is the site of extracellular digestion and it distributes nutrients about the body
o the flat, thin body allows gas exchange to occur by diffusion
o an excretory system functions as an osmotic-regulating system
o flatworms are bilaterally symmetrical; free-living forms exhibit cephalization, the development of a head region
o flatworms have a ladder-type nervous system
§ paired ganglia form a brain
§ sensory cells are in the body wall
- Free-living Flatworms: Turbellaria include freshwater planaria such as Dugesia
o planaria live in lakes, ponds, and streams and feed on small living or dead organisms
o the head is bluntly arrow-shaped; side extensions (auricles) are sensory organs to detect food and enemies
o 2 light-sensitive eyespots have pigmentation that makes them look cross-eyed
o 3 muscle layers—an outer circular, an inner longitudinal, and a diagonal—allow for varied movement
o in larger forms, cilia on ventral and lateral surfaces aid in locomotion
o numerous gland cells secrete a mucous material upon which the animal moves
o digestion: it captures food by wrapping itself around prey, entangling it in slime, and pinning it down
§ the pharynx is a muscular tube that extends through the mouth and through which food is ingested
§ three-branched gastrovascular cavity: digestion is both extracellular and intracellular
o excretion: the flame-cell system consists of a series of interconnecting canals that run length of the body on either side
§ a flame cell is a bulb-shaped cell containing a tuft of cilia inside a hollow interior; cilia sweep water into canals that empty through pores at the surface
§ it functions in both water excretion and osmotic regulation
o planaria can reproduce both sexually and asexually
§ they constrict beneath the pharynx; each half will grow into a whole animal -regeneration
§ planaria are hermaphroditic, possessing both male and female sex organs
§ planaria cross-fertilize each other
§ fertilized eggs enclosed in a cocoon hatch in 2-3 weeks into tiny worms
- Parasitic Flatworms: flukes and tapeworms
o loss of predation allows a lack of cephalization; the head carries hooks and suckers to attach to a host
o there is extensive development of reproductive system with loss of other systems
o well-developed nervous and gastrovascular systems are not needed; it does not seek out or digest prey
o flukes and tapeworms are covered by a tegument that protects them from host digestive juices
o 2 Hosts: flukes and tapeworms use a secondary (intermediate) host to travel from primary host to primary host
§ a primary host is infected with sexually mature adults; the secondary host contains the larval stage(s)
- Flukes: class Trematoda
o blood, liver, and lung flukes inhabit those organs
o fluke bodies are generally oval and elongate
o at the ³head,² an oral sucker is surrounded by sensory papillae; another sucker also helps attach
o flukes have reduced digestive, nervous, and excretory systems
o the reproductive system is well developed and they are usually hermaphroditic
o the blood fluke causes schistosomiasis
§ schistosomiasis disease is found predominantly in tropical Africa and South America where about 800,000 infected persons die each year
§ blood flukes are male or female; the female fluke deposits eggs in blood vessels around the intestines
§ the eggs migrate to the intestine and are passed out with feces
§ tiny larvae hatch in water and swim until they detect and enter a particular species of snail
§ the larvae reproduce asexually inside the snail and eventually leave the snail
§ if the larvae penetrate the skin of the human body, they begin to mature in the liver and implant in the small intestine blood vessels
§ a weakened person is more likely to die from secondary diseases
o the Chinese liver fluke requires two intermediate hosts (a snail and a fish)
§ humans become infected when they eat uncooked fish
§ adults live in the liver and deposit eggs in the bile duct, which carries eggs to the intestine
§ larval flukes must then pass through two intermediate hosts, a snail and a fish
- Tapeworms: class Cestoda
o a tapeworm head or scolex contains hooks and suckers for attachment to intestinal wall of host
o behind this head is a short neck and then a long string of proglottids
o each proglottid segment contains a full set of both male and female sex organs
o there are excretory canals but no digestive system and only rudiments of nerves
o after fertilization, proglottids become a bag of eggs; mature proglottids break off and pass out with feces
o if eggs of tapeworms are ingested by pigs or cattle, larvae become encysted in the muscle of hosts
o the covering of ingested eggs is digested away and larvae burrow through intestinal wall and travel by bloodstream to lodge and encyst in muscle
o a cyst is a hard-walled structure sheltering a larval worm
o if humans eat the meat of infected pigs or cattle and fail to cook the meat properly, they too become infected
Pseudocoelom: a body cavity that is incompletely lined with mesoderm
- body support is provided by the hydrostatic pressure of fluid in the pseudocoelom pressing against a tough cuticle
- Roundworms: Phylum Nematoda (~ 90,000 species of roundworms)
o these worms are unsegmented and have a smooth outside wall
o nematode worms are found almost everywhere: sea, fresh water, soil, etc.
o many are scavengers or parasites; some are predators
o Ascaris: have only longitudinal muscles; move by whiplike motion
§ males are smaller than females and their posterior end curves to a point
§ mating produces eggs that mature in the soil; therefore, most are limited to warmer climates
§ when eggs are swallowed, larvae burrow through intestinal wall to the liver, heart and/or lungs
§ in the lungs, the larvae molt; after 10 days they migrate up the windpipe to the throat and are swallowed
§ back in the intestine, mature worms mate and the female deposits eggs that are lost with feces
§ feces must reach mouth of the next host to complete a life cycle; therefore proper sanitation easily prevents infection
o Trichinosis is a serious infection
§ humans contract Trichinella eating raw pork with encysted larvae
§ after maturation, female adult burrows into the wall of the small intestine and produces living offspring that are carried by bloodstream to skeletal muscles where they encyst
o Filarial worms cause various diseases
§ Dirofilaria is the heartworm parasite of dogs and is a common filarial worm of temperate zones
§ elephantiasis occurs in tropical Africa
· it is caused by a filarial worm that uses the mosquito as a secondary host
· adult worms reside in and block the lymphatic vessels; ultimately this results in the limbs of an infected individual swelling to monstrous size
· it is treatable only in the early stages but not after scar tissue has blocked the lymphatic vessels
- Rotifers: Phylum Rotifera (~ 2,000 species of rotifers)
o rotifers are abundant in freshwater
o although microscopic and easily confused with protozoans, rotifers are multicellular with a pseudocoelom and organs
o a crown of cilia (corona) causes a rotating motion; this organ of locomotion also directs food to the mouth
Chapter 30: More Invertebrates (Protostomes & Deuterostomes)
Protostome Organization
- protostomes are bilaterally symmetrical, have three germ layers, the organ level of organization, the tube-within-a-tube body plan, and a true coelom
Coelom: a body cavity between the digestive tract and body wall; it is lined by mesoderm
- the coelom allows the digestive system to move independent of the body wall; internal organs can be complex
- coelomic fluid assists respiration and circulation by diffusing nutrients, and assists excretion by accumulating wastes
- this cavity may serve as a storage area for eggs and sperm
- the coelomic fluid protects internal organs against damage and temperature changes and also serves as a hydrostatic skeleton
Coelomates Are Protostomes and Deuterostomes
- protostomes include molluscs, annelids, and arthropods; they have three embryological traits
o in spiral cleavage, the cells divide without an increase in the size of cells; the fate of cells is also fixed
o the blastopore is associated with the mouth
o a coelom (schizocoelom) forms by splitting mesoderm on either side of the primitive gut
- deuterostomes include echinoderms and chordates
o they undergo radial cleavage where the new daughter cells sit on top of previous cells; the fate of these cells varies
o the blastopore is associated with the anus
o a coelom (enterocoelom) forms by the fusion of mesodermal pouches from the primitive gut
Molluscs: Phylum Mollusca (over 110,000 living species of mollusks)
- most are marine, but some are freshwater and terrestrial
- molluscs have a three-part body plan: a visceral mass, mantle, and foot
- the visceral mass contains internal organs: digestive tract, paired kidneys, and reproductive organs
- a mantle covering partly surrounds the visceral mass; it may secrete a shell and help develop the gills or lungs
- the foot is muscular and adapted for locomotion, attachment, food capture, or a combination of functions
- the radula in the mouth bears many rows of teeth and is used for grazing on food
- the nervous system consists of several ganglia connected by nerve cords
- in molluscs, the coelom is reduced and limited to the region around the heart
- most molluscs have an open circulatory system: a heart pumps hemolymph through vessels into a hemocoel
- blue hemocyanin, not red hemoglobin, is the respiratory pigment found in mollusks
- some are slow moving with no head; others are active predators with a head and sense organs
- Chitons: class Polyplacophora
o chiton shells consists of a row of eight overlapping plates
o the flat chiton foot is muscular and creeps along or clings to rocks
o they scrape algae and other plant food from rocks with a well-developed radula
- Bivalves: class Bivalvia (clams, oysters, mussels, scallops)
o ³Bivalves² are two-part shells that are hinged and close by powerful muscles
o they have no head, no radula, and little cephalization
o clams burrow with a hatchet-shaped foot; mussels use it to produce threads to attach to objects
o scallops both burrow and swim; rapid clapping of their valves releases water in spurts
o the shell is secreted by the mantle
§ shell is composed of protein and calcium carbonate with inner layer of pearl
§ pearl: shell deposits around foreign particle inserted between mantle and shell
o a compressed muscular foot projects down from shell; by expanding the tip, it pulls in the body
o the beating cilia of the gills causes water to enter the mantle cavity by way of an incurrent siphon and exit by way of an excurrent siphon
o the cilia of gills move water through mantle cavity
o gills capture particles and move them toward the mouth; the mouth leads to the stomach, which leads to the intestine, which passes through a heart and ends at the anus
o the circulatory system is open; the heart pumps hemolymph into vessels that open into the hemocoel
o the nervous system consists of three pairs of ganglia that connect the front, back, and foot
o two excretory kidneys below the heart remove ammonia waste from the pericardial cavity
o the sexes are separate; the gonad is located around coils of intestine
o some clams and annelids have the same type of larva, indicating an evolutionary relationship between molluscs and annelids
- Cephalopods: class Cephalopoda (³head-footed²) includes squids, cuttlefish, octopuses, and nautiluses
o squids and octopuses squeeze water out of the mantle cavity; the water forced out through a funnel propels them by jet propulsion
o around the head are tentacles with suckers or adhesive secretions adapted for grasping prey
o a head equipped with a powerful beak can tear prey apart
o well-developed sense organs include focusing camera-type eyes
o cephalopods, particularly octopuses, have well-developed brains with a capacity for learning
o nautiluses are enclosed in shells; squids have a reduced internal shell and octopuses lack shells
o squids and octopuses possess ink sacs; they squirt a cloud of ink to escape predators
o squids possess a vestigial shell under the mantle (the pen) which surrounds the visceral mass
o squids direct the funnel to squeeze water out to move forward or backward
o squids have a closed circulatory system where blood is always within blood vessels or the heart
§ a squid has three hearts, one pumps blood to internal organs; two pump blood to the gills
o gonads make up a large portion of the visceral mass; the sexes are separate
§ spermatophores are packets that contain sperm which a male tentacle passes to the female mantle cavity
§ after the eggs are fertilized, they are attached to substratum in strings of up to 100 eggs
- Gastropods: class Gastropoda includes snails, land slugs, whelks, conchs, periwinkles, sea hares, and sea slugs
o most are marine but some are freshwater or terrestrial
o herbivores use a radula to scrape food from surfaces; carnivorous gastropods use the radula to bore through a surface, such as a bivalve shell, to obtain food
o a developed head with eyes and tentacles projects from a coiled shell that protects visceral mass
o nudibranchs (sea slugs) and terrestrial slugs lack shells
o in development, gastropods undergo a torsion—the body is twisted to bring the anus and mantle cavity downward, forward and around to a position above head—to position the visceral mass above the foot
o in aquatic gastropod, the gills are in the mantle cavity
o in land gastropods, a mantle has blood vessels and functions as a lung when air is moved in and out through respiratory pores
o terrestrial gastropod embryonic development does not go through a swimming larval stage found in aquatic species
o for terrestrial snails, the shell not only offers protection but it also prevents desiccation
o the foot contracts in peristaltic waves from anterior to posterior; this movement is aided by a lubricating mucus that is secreted
o land snails are hermaphroditic
§ in pre-mating behavior, they meet and shoot calcareous darts into each otherıs body wall
§ each inserts a penis into the otherıs vagina; this provides sperm for future fertilization of eggs
§ eggs are deposited in soil and development proceeds without the formation of a larvae
o hermaphroditism assures that any two animals can mate–very useful in slow-moving animals
Annelids (Segmented Worms): phylum Annelida (~ 12,000 species)
- segmentation is evidenced by the rings that encircle the body; septa are internal walls that partition the coelom
- a well-developed, fluid-filled coelom and tough integument act as a hydrostatic skeleton
- segmentation may have evolved in conjunction with a hydrostatic skeleton
- using a hydrostatic skeleton, partitioning the coelom allows for independent movement of the segments so it can not only burrow but crawl on the surface
- once segmentation and the tube-within-a-tube plan appeared, each segment could specialize to perform a particular function
- the digestive system is specialized to include a pharynx, stomach, and accessory glands
- the extensive closed circulatory system has blood vessels running the length of the body and to every segment
- the nervous system has a brain connected to a ventral solid nerve cord with a ganglion in each segment
- paired nephridia in each segment collect waste material from coelom and excrete it through openings in the body wall
- Polychaete Worms: class Polychaeta
o most polychaetes are marine
o polychaetes possess parapodia and setae
§ parapodia are paddle-like appendages used in swimming and for respiration
§ setae are bristles, attached to parapodia, that help anchor polychaetes or help them move
o clam worms such as Nereis are active predators
o many have well-developed cephalization; the head has well-developed jaws, eyes, and other sense organs
o other sedentary filter feeders possess tentacles with cilia to create water currents and sort out food particles
o only during breeding do polychaetes have reproductive organs
o in Nereis, many worms coordinate the shedding of a portion of their bodies that contain either eggs or sperm; these segments float to the surface where fertilization takes place
o marine worm zygotes develop larva similar to those of marine clams; again this shows a relatedness between annelids and mollusks
- Earthworms: class Oligochaeta
o earthworms have only a few setae, protruding in pairs directly from the body
o earthworms lack both a well-developed head and any parapodia
o locomotion requires coordinated movement of body muscles and the help of setae
§ as longitudinal muscles contract, segments bulge and setae protrude to anchor into soil
§ when circular muscles contract, a worm lengthens, setae are withdrawn and the segment can be pulled forward
o earthworms live in moist soil; a moist body wall allows for gas exchange
o earthworms are scavengers that extract organic remains from soil they eat
o a muscular pharynx draws food into the mouth
o food is stored in a crop and ground up in a thick, muscular gizzard
o the dorsal surface of the intestine is expanded into a typhlosole for more surface area for digestion
o each external segment corresponds to an internal septum; a wall that separates each body segment
o a long ventral nerve cord leads from the brain to ganglionic swellings and lateral nerves in each segment
o the excretory system involves nephridia
§ the coiled nephridia tubules in each segment have two openings: one is a ciliated funnel that collects coelomic fluid, and the other is an exit through the body wall
§ between the two openings, a coiled nephridia tubule removes waste from blood vessels
o the dorsal blood vessel moves red blood anteriorly; 5 pairs of hearts pump blood to a ventral vessel
o reproduction
§ earthworms are hermaphroditic
§ the male organs are the testes, sperm ducts, and seminal vesicles
§ the female organs are the ovaries, oviducts, and seminal receptacles
§ mating involves aligning parallel to each other facing opposite directions to exchange sperm
§ each possesses a clitellum that secretes mucus that slides off, forming a slime tube that protects the sperm and eggs from drying out
§ the slime tube forms a cocoon around the fertilized eggs as they develop
§ embryonic development lacks a larval stage
o comparison of the clam worm and earthworm show adaptations to marine and terrestrial life through presence or absence of cephalization, parapodia, a slime tube cocoon, and trochophore larvae
- Leeches: class Hirudinea
o most are freshwater species but a few are marine or terrestrial
o they lack setae and each body ring has several transverse grooves
o leeches possess a small anterior sucker around the mouth and a larger posterior sucker
o although some are free-living predators, most feed on body fluids
o leeches keep blood from coagulating by hirudin, an anticoagulant in their saliva
Arthropods: phylum Arthropoda (over 1,000,000 species)
- they are considered highly successful because they have adapted to so many habitats
- arthropods have freely movable jointed appendages
- arthropods have a strong but flexible outer covering called an exoskeleton composed mainly of chitin (a nitrogenous polysaccharide )
o the exoskeleton serves for protection, attachment for muscles, locomotion, and prevention of desiccation
o because the exoskeleton is hard and nonexpandable, arthropods must molt (shed) the exoskeleton to grow larger
- some segments of arthropods have fused into regions (head, thorax, and abdomen)
o trilobites (Cambrian period) had a pair of appendages on each segment; modern arthropod appendages specialize for walking, swimming, reproduction, etc.
- arthropods have a well-developed nervous system
o the head has a brain is connected to a ventral solid nerve cord and sensory organs
o compound eyes have many complete visual units; each collects light independently
o in simple eyes, a single lens brings the image to focus into many receptors, each of which receives only a portion of the image
- arthropods use a variety of respiratory organs
o marine forms use gills with thin vascularized tissue specialized for gas exchange
o terrestrial forms use book lungs (e.g., spiders) or tracheae air tubes (e.g., insects)
- metamorphosis is a drastic change in form and physiology that occurs as a larva becomes an adult
o reduction in competition between larvae & adults (live in different environments) allows more members of the species to exist at one time
- Crustaceans: subphylum Crustacea (about 40,000 species)
o crustaceans are successful and mostly marine arthropods
o the head usually bears compound eyes and five pairs of appendages
o the first two appendages are antennae and antennules that have sensory functions
o the next three pairs (mandibles, first and second maxillae) lie behind the mouth and are used in feeding
o biramous appendages have two branches; one branch is a gill and the other is the leg branch
o copepods and krill feed on algae; numerous, they are an important link in marine food chains
o barnacles have a thick, heavy shell and are sessile
§ stalked barnacles attach by a stalk; stalkless barnacles attach directly to shell
§ barnacles begin as free-swimming larvae and become sessile on wharf pilings, rocks, etc. and extend feathery structures (cirri) to filter feed
o decapods include shrimps, lobsters, crabs, crayfish, etc.
§ their thorax has five pairs of walking legs; the first pair may be modified as claws
§ usually respiratory gills are above the walking legs
§ the nonsegmented carapace covers the fused head and thorax (cephalothorax)
§ abdominal segments have a pair of swimmerets, small paddlelike structures
§ the first pair of swimmerets in a male are stronger to pass sperm to the female
§ the last tail segments are the uropod and the telson, which together make a fan-shaped tail
§ a crayfish awaits prey and uses its claws to carry it to the mouth
§ the crayfish stomach has two main regions: the anterior gastric mill with chitinous teeth grinds food, and posterior region filters coarse particles before absorption in the digestive glands
§ green glands in the head area excrete metabolic wastes through a duct at the base of the antennae
§ the coelom is reduced in arthropods and forms the space about the reproductive system
§ heart pumps hemolymph containing bluish hemocyanin into a hemocoel where it washes around the organs
§ a brain is connected to a ventral nerve cord; ganglia give off lateral nerves
§ the sexes are separate in crayfish
· the male has a coiled sperm duct that opens to the outside at the base of fifth walking leg
· the femaleıs ovaries open at the base of the third walking legs, with a fold between the fourth and fifth pair of legs serving as a seminal receptacle
· following fertilization, eggs are attached to the swimmerets of the female
- Uniramians: subphylum Uniramia (millipedes, centipedes, and insects)
o the uniramous appendages attached to the thorax and abdomen have only one branch—the leg branch
o the head appendages include one pair of antennae, one pair of mandibles, and one or two pairs of maxillae
o uniramia live on land and breathe by air tubes called tracheae
- Insects: superclass Insecta (over 900,000 species; this exceeds all other animal species combined)
o most insects live on land; some are secondarily aquatic
o insect body is divided into a head, thorax, and abdomen
§ the head bears sense organs and mouthparts
§ the thorax bears three pairs of legs and one or two pairs of wings; the wings provide advantages in escaping enemies, finding food, and mating
§ the abdomen contains most of the internal organs
o the exoskeleton of an insect is lighter and contains less chitin than that of many other arthropods
o grasshoppers have adaptations as herbivorous insects
§ the third pair of legs is suited to jumping
§ the front wings are protective and leathery; the thin hind pair of wings fold up
§ each side of the first abdominal segment has a tympanum for sound wave reception
§ two paired projections form an ovipositor in females used to dig a hole for laying eggs
§ the grasshopper digestive system is complex
· the mouth mechanically digests food; salivary secretions begin digestion
· the crop temporarily stores food
· a gizzard finely grinds the food
· digestion is completed in stomach; gastric ceca assist nutrient absorption
§ the excretory system consists of Malpighian tubules extending into hemocoel
· nitrogenous wastes are collected and excreted into the digestive system
· formation of solid nitrogenous wastes (uric acid) conserves water
§ the respiratory system begins with openings in exoskeleton called spiracles
· air enters into small tubules called tracheae
· tracheae branch many times until they reach moist areas of gas absorption
· air movement through this tracheal system is assisted by air sacs
· air enters anterior four spiracles and exits by the posterior six
· using tracheae is a factor that limits the size of insects
§ the circulatory system has a slender, tubular heart that pumps colorless hemolymph into a hemocoel where it circulates before returning to the heart (insect hemolymph has no pigment since the tracheal system transports gases)
§ reproduction is adapted to life on land
· a male grasshopper has a penis to transfer sperm to the female
· internal fertilization protects the gametes from drying out
· female grasshoppers deposit eggs in the ground with her ovipositor
§ grasshoppers undergo incomplete metamorphosis; the immature nymph resembles adult
§ other insects undergo complete metamorphosis; the wormlike larvae reorganize into different adults
§ some species (e.g., bees and ants) exhibit colonial social behavior
§ entomology, the study of insects, is a major field of biology
o crustacea are primarily marine or freshwater organisms, use gills and an oxygen carrying pigment, and excrete liquid ammonia wastes; insects are primarily terrestrial or freshwater organisms, use tracheae and lack respiratory pigments, and excrete solid wastes
- Centipedes and Millipedes (Superclass Myriapoda)
o the class Chilopoda includes about 2,500 species of centipedes
§ the body is composed of a head and trunk with many segments; each segment has one pair of legs
§ centipedes are carnivorous; the head bears antennae and mouthparts with poison fangs
o the class Diplopoda includes about 10,000 species of millipedes
§ millipedes have a cylindrical segmented body
§ some body segments are fused with two pair of legs on each resulting segment
§ they possess more legs than centipedes, but not one thousand as the name states
§ millipedes dwell in the soil, feeding on dead organic matter
- Chelicerates: subphylum Chelicerata (spiders, scorpions, ticks, mites, marine horseshoe crabs, and sea spiders)
o the first pair of appendages are chelicerae, the second pair are pedipalps, and the next four pairs are walking legs
§ chelicerae are appendages that function as feeding organs
§ pedipalps are feeding or sensory structures
o all appendages attach to a cephalothorax, a fusion of head and thoracic regions
o the head lacks antennae, mandibles, or maxillae appendages
o the marine horseshoe crab (genus Limulus) is common along the east coast of North America
§ they scavenge sandy and muddy substrates for annelids, molluscs and other invertebrates
§ the anterior shield is a horseshoe-shaped carapace with two compound eyes
§ a long, unsegmented telson tail projects to the rear
§ they possess book gills that resemble the pages in a book
o scorpions are arachnids and are the oldest terrestrial arthropods known from fossils
§ they are nocturnal and spend the day hidden under a log or rock
§ pedipalps are large pincerlike appendages; the abdomen ends in a stinger containing venom
o ticks and mites number over 25,000 species and outnumber all other arachnids
§ ticks are parasites that suck blood and sometimes transmit diseases
§ chiggers are larvae of certain mites and feed on the skin of vertebrates
o spiders have a narrow waist separating cephalothorax from abdomen
§ spiders have numerous simple eyes rather than compound eyes
§ spider chelicera are modified as fangs with ducts leading from poison glands
§ the abdomen has silk glands; they may spin a web to trap prey
§ invaginations of the body wall form lamellae (pages) of book lungs
Echinoderms: phylum Echinodermata (~ 6,000 species of echinoderms) are in
- modern echinoderms (along with the phylum Chordata) are deuterostomes
- all echinoderms have an endoskeleton consisting of calcareous plates bearing spines
- class Crinoidea includes 600 species of crinoids; includes stalked sea lilies and motile feather stars
- class Holothuroidea has 1,500 species of sea cucumbers; they have a long leathery body and feed by tentacles about the mouth
- class Echinoidea has about 950 species of sea urchins and sand dollars; they possess spines for locomotion, defense, and burrowing
- class Ophiuroidea has about 2,000 species of brittle stars; they have a central disk from which long, flexible arms radiate
- class Asteroidea contains 1,500 species of sea stars (starfishes)
o sea stars are common along rocky coasts; they feed on clams, oysters, and other bivalves
o the five-rayed body has an oral (mouth) and aboral (upper) side
o various structures project through the body wall:
§ spines project from the endoskeletal plate through the thin dermis
§ pincerlike pedicellarie keep the surface free from particles
§ gas exchange is conducted by skin gills
§ on the oral surface, each arm has a groove lined with tube feet
o sea stars feed on bivalves by everting the cardiac stomach - tube feet open the shell slightly and the cardiac stomach secretes enzymes to begin digestion; the pyloric stomach completes digestion
o a short intestine opens at the anus on the aboral side
o in each arm is a developed coelom containing paired digestive glands and male or female gonads
o the nervous system is a central ring with radial nerves extending into each arm
o a light-sensitive eyespot is at the tip of each arm
o locomotion depends upon a water vascular system: water enters on the aboral side through a sieve plate (madreporite), passes through a stone canal to a ring canal and into radial canals in each arm
§ the radial canals feed into lateral canals extending into tube feet; each tube foot has an ampulla that contracts to force water into the tube foot
§ when the foot touches a surface, the center withdraws forming a suction and adhering to surfaces
o echinoderms lack a complex respiratory, excretory, or circulatory system
§ nitrogenous wastes diffuse through the coelomic fluid and across a body wall
§ cilia on the peritoneum lining the coelom keep the coelomic fluid moving
o sea stars reproduce both sexually and asexually
§ if the sea star is fragmented, each fragment can regenerate a whole animal
§ sea stars spawn and release either eggs or sperm at the same time
§ the bilateral larvae undergoes a metamorphosis to become a radially symmetrical adult
Chapter 31: Vertebrates
The Chordates: phylum Chordata (45,000 species)
- chordates (like echinoderms) are deuterostomes
- at some time during their life, all chordates have 4 basic characteristics:
o notochord: a supporting rod located dorsally just below the nerve cord; it is replaced by the vertebral column in vertebrates
o dorsal hollow nerve cord: a cord containing a fluid-filled canal; in vertebrates, this is the spinal cord and is protected by vertebrae
o pharyngeal pouches: openings functioning in feeding, gas exchange, or both
§ they are seen only during embryonic development in most vertebrates
§ in invertebrate chordates, fish, and amphibian larvae, they become gills
§ in terrestrial vertebrates, the pouches are modified for various purposes
§ in humans, the first pair of pouches become the auditory tubes, the second become tonsils, and the third and fourth pairs become the thymus and parathyroid glands
o a postanal tail extends beyond the anus; in some, this only appear in embryos
- Invertebrate Chordates
o the notochord persists and is never replaced by the vertebral column in these species
o lancelets: genus Branchiostoma in subphylum Cephalochordata (23 species)
§ an elongated body resembles the lancelet, a two-edged surgical knife
§ they inhabit shallow coastal waters and filter feed partly buried in sand
§ lancelets retain the four chordate characteristics as adults
§ the notochord extends from head to tail, accounting for the subphylum name
§ they have segmented muscles and the dorsal hollow nerve cord has periodic branches
o tunicates (sea squirts): subphylum Urochordata (1,250 species)
§ adults have a body composed of an outer tunic; an excurrent siphon squirts out water when it is disturbed
§ larvae are bilaterally symmetrical and have the four chordate characteristics
§ the larvae undergo metamorphosis to develop into sessile adults
§ water passes into a pharynx and out numerous gill slits, the only chordate characteristic that remains in adults
§ cilia lining the inside of the pharynx create a current to move water through
§ microscopic particles adhere to mucous secretion in the pharynx and are eaten
§ the urochordate larva may have been ancestral to vertebrates
Vertebrates: subphylum Vertebrata (43,700 species)
- vertebrates have all four chordate characteristics at some time during their lives
- the embryonic notochord is replaced by a vertebral column with individual vertebrae surrounding a dorsal hollow nerve cord
o the vertebral column is part of a flexible, strong endoskeleton that is also evidence of segmentation
- the vertebrate skeleton is living tissue (either cartilage or bone) that grows with the animal
- the endoskeleton and muscles together permit rapid and efficient movement
- the pectoral and pelvic fins of fish evolved into jointed appendages allowing vertebrates to move onto land
- a skull is anterior component of main axis of vertebrate endoskeleton; it encases the brain
- the high degree of cephalization in vertebrates is accompanied by complex sense organs
o the eyes developed as outgrowths of the brain
o ears - equilibrium devices in water - function as sound-wave receivers in land vertebrates
- vertebrates possess a complete digestive system and a large coelom
- the circulatory system is closed and the blood is contained within blood vessels
- gills or lungs provide efficient gas exchange
- the kidneys efficiently excrete nitrogenous waste and regulate water
- reproduction is usually sexual with separate sexes
- evolution of the amnion allowed reproduction to take place on land
- the development of the placenta in mammals allowed development in the uterus of the female
- Fishes are aquatic, gill-breathing vertebrates that usually have fins and skin covered with scales
- Jawless Fishes (agnathans): superclass Agnatha (63 species)
o lampreys and hagfishes are modern jawless fishes and they lack a bony skeleton, have smooth nonscaly skin and have cylindrical bodies and are up to a meter long
o hagfishes are scavengers feeding on soft-bodied invertebrates and dead fishes
o many lampreys are filter feeders similar to their ancestors
o parasitic lampreys have a round muscular mouth equipped with teeth; they attach themselves to fish and suck nutrients from the hostıs circulatory system
o marine parasitic lampreys entered the Great Lakes and devastated the trout population in the1950s
- Fishes with Jaws: animals beyond this point are gnathostomates, animals with jaws, the tooth-bearing bones of the head
o placoderms are extinct jawed fishes of the Devonian period that were armored with heavy plates and had strong jaws
o like extant fishes, they had paired pectoral and pelvic fins for balance and maneuvering well in water; this helps predation
- Cartilaginous Fishes (sharks, rays, skates): class Chondrichthyes (850 species)
o they have a cartilaginous skeleton rather than bone
o five to seven gill slits are on both sides of the pharynx; they lack the gill covers found on bony fish
o their body is covered by epidermal placoid (toothlike) scales
o the teeth of sharks are enlarged scales; there are many rows of replacement teeth growing behind the front teeth
o they have three well developed senses to detect electric currents in water, pressure (a lateral line system), and smell
o the largest sharks are filter feeders, not predators; the basking and whale sharks eat tons of crustacea
o most sharks are fast, open-sea predators; a great white shark eats dolphins, sea lions and seals
o rays and skates live on the ocean floor; their pectoral fins are enlarged into winglike fins and they swim slowly
o stingrays have a venomous spine
o electric rays feed on fish that have been stunned with an electric shock that may reach over 300 volts
o sawfish rays have a large anterior ³saw² used to slash through schools of fish
- Bony Fishes: class Osteichthyes (~ 20,000 species)
o bony fishes have a bony skeleton; most are ray-finned with thin, bony rays supporting the fins
o a few lobe-finned fishes are related to ancestors of amphibians
o the ray-finned fishes include our familiar fishes
§ they are the most successful and diverse of vertebrates
§ they vary from filter feeders to predaceous carnivores
§ their skin is covered by scales formed of bone
§ the gills do not open separately but instead are covered by an operculum
§ the swim bladder is a gas-filled sac whose pressure can be altered to regulate buoyancy and depth
§ closed circulatory system with 2-chambered heart
§ salmon, trout, and eels migrate between fresh and salt water but adjust their kidney and gill function
§ fish sperm and eggs are usually shed into water
§ for most fish, the fertilization and embryonic development occur outside the femaleıs body
o the lobe-finned fishes include six species of lungfishes and one species of coelacanth; their fleshy fins are supported by central bones
§ lungfishes live in stagnant water or ponds that dry up; found in Africa, South America and Australia
§ coelacanths live in deep oceans; once considered extinct, more than 200 have been captured since 1938 near the Comoros Islands, with recent finds near Malaysia
- Amphibians: class Amphibia (3,900 species )
o all animals studied from this point on have four limbs and are tetrapods
o the lobe-finned fishes of Devonian are ancestral to amphibians
o land animals use limbs to support their body since air is less buoyant than water
o two hypotheses describe evolution of amphibians from lobe-finned fishes
§ lobe-finned fishes that could move from pond-to-pond had an advantage over those that could not
§ the supply of food on land and the absence of predators promoted adaptation to land
o the first amphibians diversified during Carboniferous period which is known as the Age of the Amphibians
o diversity of amphibians
§ modern amphibians include three groups: frogs and toads, salamanders and newts, and the caecilians
§ salamanders and newts have a long body and tail, and two pairs of legs; they resemble the earliest fossil amphibians
§ salamanders and newts are carnivorous, feeding on insects, snails, etc.
§ salamanders practice internal fertilization; the males produce a spermatophore that females pick up with the cloaca
§ frogs and toads are tailless as adults; hind limbs are specialized for jumping
§ frogs and toads have the head and trunk fused; frogs live near or in fresh water while toads live in damp places away from water
§ caecilians are legless; most burrow in soil and feed on worms, etc.
§ reproduction involves a return to the water; ³amphibian² refers to this need to return to water from land
· they shed eggs into the water for external fertilization
· generally, amphibian eggs are protected by a coat of jelly but not a shell
· the young hatch into aquatic larvae with gills (tadpoles)
· the aquatic larvae usually undergo metamorphosis to develop into a terrestrial adult
o anatomy and physiology of amphibians
§ tongue is used for catching prey
§ the eyelids keep their eyes moist
§ amphibian ears are adapted for detecting sound waves; in turn, the larynx produces calls
§ the amphibian brain is larger than that of fishes; their cerebral cortex is more developed
§ amphibians usually have small lungs supplemented by gas exchange across porous skin
§ the single-loop circulatory path of fish is replaced by a closed double-loop circulatory system; however oxygen-rich blood mixes with some oxygen-poor blood
§ a three-chambered heart with a single ventricle pumps mixed blood before and after it has gone to the lungs
§ amphibian skin is thin, smooth, and nonscaly, and contains numerous mucous glands; this skin plays an active role in osmotic balance and respiration
§ some skin glands secrete poisons; those tropical species often have brilliant warning coloration
§ amphibians are ectothermic, depending upon the environment to regulate body temperature
§ if winter temperature drops too low, temperate ectotherms become inactive and enter torpor
- Reptiles: class Reptilia (~ 6000 species)
o reptiles were the first vertebrates to practice internal fertilization through copulation and to lay eggs that are protected by a leathery shell
o the amniote egg contains extraembryonic membranes that are not a part of the embryo and are disposed of after development
o they protect the embryo, remove nitrogenous wastes, and provide oxygen, food, and water
o the amnion is one extraembryonic membrane; it fills with fluid to provide a ³pond² for embryo to develop
o reptiles evolved from amphibian ancestors by Permian period
o the first reptiles (stem reptiles) gave rise to several lineages; each was adapted to a different way of life
o dinosaurs varied in size and behavior; some had a bipedal stance and gave rise to birds
o reptiles dominated earth for about 170 million years during the Mesozoic era; then most died out
o Luis and Walter Alvarez proposed the impact theory of dinosaur extinction: a large meteorite or comet at the end of the Cretaceous period could have set off earthquakes and fires, raising enough dust and smoke to block out the sun; an iridium layer, a mineral common in meteorites, occurs in rocks at the end of this period
o diversity of reptiles:
§ most reptiles today live in the tropics or subtropics; lizards and snakes live on soil; turtles, crocodiles and alligators live in water
§ tuataras are lizardlike and identical to fossils 200 million years old
§ crocodiles and alligators are largely aquatic, feeding on fishes and other animals
· their powerful jaws have numerous teeth; a muscular tail is both a paddle to swim and a weapon
· male crocodiles bellow to attract mates; males of some species protect the eggs and young
§ turtles have a heavy shell fused to the ribs and to the thoracic vertebrae
· turtles lack teeth but use a sharp beak
· sea turtles must return to lay eggs onshore
§ lizards have four clawed legs and are carnivorous
· marine iguanas on Galapagos are adapted to spend long times in the sea
· chameleons live in trees, have a long sticky tongue to catch insects, and change color
· australian frilled lizards have a collar to scare predators
§ snakes evolved from lizards and lost legs as an adaptation to burrowing
· their jaws can readily dislocate to engulf large food
· a tongue collects airborne molecules to transfer them to Jacobsonıs organ for tasting
· some snakes are poisonous and have special fangs to inject venom
§ reptiles have a thick, scaly skin that is keratinized and is impermeable to water
· keratin is the protein that is also found in hair, fingernails, and feathers
· reptileıs protective skin prevents water loss but it also requires several molts a year
§ reptile lungs are more developed than in amphibians; air rhythmically moves in and out of the lungs due to an expandable rib cage, except in turtles
§ most have a nearly four-chambered heart; crocodiles have a completely four-chambered heart; oxygen-rich blood is more fully separated from oxygen-poor blood
§ well-developed kidneys excrete uric acid; therefore, less water is lost in excretion
§ reptiles are ectothermic
· they require a fraction of the food per body weight of birds and mammals
· they are behaviorally adapted to warm their body temperature by sunbathing
- Birds: class Aves (~ 9,000 species)
o birds lack teeth and have only a vestigial tail but their relationship to reptiles shows in the scales on their legs, claws on their toes, and a beak
o birds also lay an egg, but it is hard-shelled rather than leathery
o the ancestry of birds is in dispute; some biologists consider them related to bipedal dinosaurs
o bird anatomy is closely related to its ability to fly
o bird forelimbs are modified as wings for flying with hollow, light bones laced with air cavities
o a beak composed of keratin has replaced jaws equipped with teeth
o a keeled breastbone anchors muscles used in flight
o bird respiratory air sacs are extensive, even extending into some larger bones
§ using a one-way flow of air, air sacs maximize gas exchange and oxygenation of blood
§ efficient supply of oxygen to muscles is vital for level of muscle activity needed for flight
o birds possess a four-chambered heart; this double-loop circulatory system separates oxygen-rich and oxygen-poor blood
o birds are endothermic; they have ability to maintain a constant, relatively high body temperature
§ homeothermy enables an animal to be continuously active in cold weather
§ feathers may have evolved for insulation and later became adapted for flight
o flight requires well-developed sense organs and nervous system
§ birds have very acute vision and excellent muscle reflexes
§ bird muscle reflexes are excellent
§ bird migration allows use of widespread food sources; an enlarged portion of the brain is responsible for instinctive behaviors
o bird classification is based on beak and foot types, and some on habitats and behaviors
§ birds of prey have notched beaks and sharp talons
§ shorebirds have long slender bills and long legs
§ waterfowl have webbed toes and broad bills
- Mammals: class Mammalia (over 4,800 species)
o mammals evolved during the Mesozoic era from therapsids, extinct mammallike reptiles
o the mammal skull is bigger, their teeth are differentiated into molars and premolars, and the vertebral column provides more movement
o true mammals appeared during the Jurassic period, about the same time as the first dinosaurs
§ the first mammals were small, about the size of mice
§ some of the earliest mammalian groups were monotremes and marsupials
§ placental mammals evolved later to occupy habitats vacated by dinosaurs
o the chief characteristics of mammals are hair and mammary glands
o mammals are endothermic; they produce heat and maintain a constant body temperature
o many adaptations of mammals are related to temperature control
o hair provides insulation against heat loss and allows mammals to be active in cold weather
o gas exchange is efficiently accomplished by lungs
o mammals possess a four-chambered heart and a double-loop circulatory system
o mammary glands enable females to feed young without deserting them to obtain food
o nursing creates a bond between mother and offspring to ensure parental care while the young are helpless
o in most mammals, the young are born alive after a period of development in uterus
o Mammals That Lay Eggs
§ monotremes are mammals that have a cloaca and lay hard-shelled amniote eggs
§ they are represented by the duckbill platypus and the spiny anteater of Australia
§ a female duckbill platypus lays her eggs in a burrow in the ground where she incubates them
§ after hatching, young lick milk seeping from modified sweat glands on the abdomen
§ the spiny anteater has a pouch formed by swollen mammary glands and muscle; the egg moves from cloaca to pouch and hatches; the young remain for 53 days and live in burrow where the mother feeds them
o Mammals That Have Pouches
§ marsupials begin development inside the motherıs body but are then born in a very immature state
§ the newborns crawl up into a pouch on their motherıs abdomen
§ inside a pouch they attach to the nipples of the motherıs mammary glands and continue to develop
§ today, most marsupials are found in Australia where they underwent adaptive radiation for several million years without competition from the placental mammals, only introduced recently
o Mammals That Have Placentas
§ placental mammals use a placenta, an organ of exchange between maternal and fetal blood
§ the placenta supplies nutrients to and removes wastes from the blood of developing offspring
§ a placenta also allows a mother to move about while the offspring develop
§ the placenta enables young to be born in a relatively advanced stage of development
§ placental mammals are very active animals; they possess acute senses and a relatively large brain
§ the brains of placental animals have cerebral hemispheres proportionately larger than other animals
§ the young go through a long period of dependency on their parents after birth
§ today, placental mammals populate all of the continents except Antarctica
§ most are terrestrial, but some are aquatic, and bats can fly
o Classification of mammals is based on mode of locomotion and the method of obtaining food
§ the order Perissodactyla includes 17 species of horses, zebras, tapirs, and rhinoceroses and the order Artiodactyla includes 185 species of pigs, cattle, deer, buffaloes, giraffes, etc.
· both orders are hoofed animals
· they have elongated limbs adapted for running across open grassland
· they are herbivorous and have large grinding teeth
§ order Carnivora contains about 270 species of dogs, cats, bears, raccoons, and skunks
· the canines of meat eaters are large and conical
· aquatic carnivores such as seals and sea lions must return to land to reproduce
§ the order Primates contains 180 species of lemurs, monkeys, gibbons, chimpanzees, gorillas, and humans
· typical primates are tree-dwelling fruit eaters; some are ground dwellers
· their digits have nails, not claws; the thumb is more opposable
· primates, particularly humans, have well-developed brains
§ the order Cetacea includes about 80 species of whales and dolphins
· they lack substantial hair or fur
· blue whales are the largest animal ever to live on this planet are baleen whales that strain plankton from the water
· toothed whales feed on fish and squid
§ the order Chiroptera contains 925 species of nocturnal bats
· wings are layers of skin and connective tissue stretched between the elongated bones of all fingers but first
· many species use echolocation to locate their usual insect prey
· some bats also eat birds, fish, frogs and plant tissues
§ order Rodentia is rodents (e.g., mice, rats, squirrels, beavers, and porcupines)
· this is largest order with 1,760 species
· rodents have incisors that grow continuously
· most rodents eat seeds but some are omnivorous or eat mainly insects
§ only two extant species are in order Proboscidea: the elephants
· elongated and muscularized upper lip and nose form a prehensile trunk
· they are herbivores and are largest living land mammals
§ order Lagomorpha includes 65 species of rabbits, hares, and pikas
· they resemble rodents but have two pairs of continuously growing incisors
· their hind legs are longer than their front legs and they are herbivores