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
§