Chapter 33: An Introduction to Invertebrates

Term DefinitionInvertebrates animals that lack a backbone; 95% of known

animal speciesSponges basal animals that lack true tissues and organs;

animals in the phylum Porifera; sedentary and live in marine waters or fresh water; suspension feeders—capturing food particles suspended in the water that passes through their body; water is drawn through pores ostia into a cavity, the spongocoel and out through an opening, the osculum; consists of a gelatinous noncellular mesohyl layer between two cell layers; most are hermaphrodites

Choanocytes flagellated collar cells; generate a water current through the sponge, filter food particles, and ingest suspended food

Amoebocytes found in the mesohyl and play roes in digestion and structure

Cnidarians an ancient phylum of eumetazoans; one of the oldest groups in the clade Eumetaoa; exhibit a relatively simple diploblastic, radial body plan; carnivores that use tentacles to capture prey

Eumetazoa clade of animals with true tissues; all animals except sponges and a few other groups

Gastrovascular cavity a central digestive compartment in a sac which is the basic body plan of a cnidarians

Polyp adheres to the substrate by the aboral end of its body

Medusa a bell-shaped body with its mouth on the underside; do not attach to the substrate but move freely

Cnidocytes unique cells that function in defense and capture of prey

Nematocysts specialized organelles within cnidocytes that eject a stinging thread

Hydrozoans most alternate between polyp and medusa formsHydra a freshwater cnidarians, exists only in polyp form

and reproduces asexually by buddingJellies (medusae) the prevalent form of the life cycle in the class

ScyphozoaCubozoans includes box jellies and sea wasps; medusa is

box-shaped and has complex eyes; often have highly toxic cnidocytes

Anthozoans includes the corals and sea anemones; occur only as polyps

Corals often form symbioses with algae and secrete a hard external skeleton

Lophotrochozoans a clade identified by molecular date; have the widest range of animal body form; some develop a lophophore for feeding, others pass through a trochophore larval stage, and a few have neither

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feature; include flatworms, rotifers, ectoprocts, brachiopods, molluscs, and annelids

Bilaterian have bilateral symmetry and triploblastic development; most have a coelom and a digestive tract with two openings

Flatworms Platyhelminthes; live in marine, freshwater, and damp terrestrial habitats; undergo triploblastic development; acoelomates; flattened dorsoventrally and have a gastrovascular cavity with one opening; gas exchange takes place across the surface

Protonephridia regulate the osmotic balanceCatenulida “chain worms”; reproduce asexuallyby buddingRhabditophora more diverse and include both free-licing and

parasitic speciesPlanarians best-known rhabditophorans; live in fresh water

and prey on smaller animals; have light-sensitive eyespots and centralized nerve nets; nervous system is more complex than the nerve nets of cnidarians; hermaphroditesl can reproduce sexually, or asexually through fission

Parasitic rhabditophorans live in or on other animals; trematodes and tapeworms

Trematodes parasitize a wide range of hosts, and most have complex life cycles with alternating sexual and asexual stages

Tapeworms parasites of vertebrates and lack a digestive system; absorb nutrients from the host’s intestine

Scolex contains suckers and hooks for attaching to the host

Proglottids units that contains sex organs and form a ribbon behind the scolex

Rotifers tiny animals that inhabit fresh water, the ocean, and damp soil; smaller than many protists but are truly multicellular and have specialized organ systems

Alimentary canal a digestive tube with a separate mouth and anus that lies within a fluid-filled pseudocoelom

Parthenogenesis reproduction in rotifers; females produce offspring from unfertilized eggs

Lophophore a crown of ciliated tentacles around their mouthLophophorates have a true coelom; Ectoprocta and BrachiopodaEctoprocts Bryozoans; sessile colonial animals that

superficially resemble plantsBrachiopods superficially resemble clams and other hinge-

shelled molluscs, but the two halves of the shell are dorsal and ventral rather than lateral as in clams; marine and attached to the seafloor by a stalk

Molluscs includes snails and slugs, oysters and clams, and octopuses and squids; most are marine, though some inhabit fresh water and some snails and slugs are terrestrial; soft-bodied animals, but most

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are protected by a hard shell; animal group with the largest number of recent extinctions

Foot Visceral mass Mantle Mantle cavityRadulaTrocophore ciliated larval stagePolyplacophora chitonsGastropoda snails and slugsBivalvia clams, oysters, and other bivalvesCephalopoda squids, octopuses, cuttlefish, and chambered

nautilusesChitons oval-shaped marine animals encased in an armor

of eight dorsal plates; use their foot like a suction cup to grip rock, and their radula to scrape algae off the rock surface

Gastropods ¾ of all living species; most are marine, but many are freshwater and terrestrial species

Torsion most distinctive characteristic of gastropods; causes the animal’s anus and mantle to end up above its heads; different from the coiling of a shell

Bivalves marine and include many species of clams, oysters, mussels, and scallops; have a shell divided into two halves drawn together by adductor muscles; some have eyes and sensory tentacles along the edge of their mantle

Cephalopods carnivoreswith beak-like jaws surrounded by tentacles of their modified foot

Nautilus small group of shelled cephalopods that survives today

Ammonites shelled cephalopods were common but went extinct at the end of the Cretaceous 65.5 million years ago

Annelids have bodies composed of a series of fused rings; coelomates

Polychaeta have paddle-like parapodia that work as gills and aid in locomotion; most are marine

Oligochaeta earthworms and their relatives, and leeches; named for relatively sparse chaetae, bristles made of chitin

Earthworms eat through soil, extracting nutrients as soil moves through the alimentary canal; hermaphrodites but cross-fertilize; some reproduce asexually by fragmentation

Leeches most species live in fresh water; some are marine or terrestrial; include predators of invertebrates, and parasites that suck blood; secrete a chemical called hirudin to prevent blood from coagulating

Ecdysozoans the most species-rich animal group; covered by a tough coat called a cuticle—shed or molted through a process called ecdysis

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Nematodes roundworms; found in most aquatic habitats, in the soil, in moist tissues of plants, and in body fluids and tissues of animals; have an alimentart canal, but lack a circulatory system; reproduction is usually sexual, by internal fertilization

Caenorhabditis elegans a model organism in researchTrichinella spiralis can be acquired by humans from undercooked

porkArthropods found in nearly all habitats of the biosphere; body

plan consists of a segmented body, hard exoskeleton, and jointed appendages; evolution characterized by a decrease in the number of segments and an increase in appendage specialization; some appendages are modified for functions; body is completely covered by the cuticle, an exoskeleton made of layers of protein and the polysaccharide chitin

Open circulatory system hemolymph is circulated into the spaces surrounding the tissues and organs

Chelicerates sea spiders, horseshoe crabs, scorpions, ticks, mites, and spiders; named for clawlike feeding appendages (chelicerae);

Eurypterids earliest cheliceriforms; water scorpionsMyriapods centipedes and millipedes; terrestrial; have jaw-

like mandiblesMillipedes eat decaying leaves and plant matter; have many

legs, with two pairs per trunk segmentCentipedes carnivores; have one pair of legs per trunk

segmentHexapods insects and relativesCrustaceans crabs, lobsters, shrimps, barnacles, and many

others; remain in marine and freshwater environments; typically have branched appendages that are extensively specialized for feeding and locomotion; small crustaceans exchange gases through the cuticle; larger crustaceans have gills

Arachnids most modern cheliceriforms; spiders, scorpions, ticks, and mites

Book lungs respiratory organs where gas exchange occursSilk a liquid protein produced by many spiders from

specialized abdominal glandsInsects Hexapoda; has more species than all other forms

of life combined; live in almost every terrestrial habitat and in fresh water

Incomplete metamorphosis the young (nymphs) resemble adults but are smaller and go through a series of molts until they reach full size

Complete metamorphosis have larval stages known by such names as maggot, grub, or caterpillar; looks entirely different from the adult stage

Isopods include terrestrial, freshwater, and marine speciesDecapods all relatively large crustaceans and include

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lobsters, crabs, crayfish, and shrimpCopepods among the most numerous of all animalsBarnacles a group of mostly sessile crustaceans; have a

cuticle that is hardened into a shellEchinoderms include sea stars and sea urchins; slow-moving or

sessile marine animals; a thin epidermis covers an endoskeleton of hard calcareous plates; have a unique water vascular system—a network of hydraulic canals branching into tube feet that function in locomotion and feeding; sexual reproduction is external

Chordates include the vertebratesDeuterostomes echinoderms and chordates; share developmental

characteristics (radial cleavage and formation of the anus from the blastopore); defined primarily by DNA similarities

Asteroidea sea stars and sea daisiesSea stars have multiple arms radiating from a central disk;

feed on bivalves by prying them open with their tube feet, everting their stomach, and digesting their prey externally with digestive enzymes; can regrow lost arms

Tube feet on the undersurface of each arm; grip substrate with adhesive chemicals

Sea daisies 1986; only three species are known; live on submerged wood and absorb nutrients through a membrane that surrounds their body

Ophuroidea brittle starsBrittle stars have a distinct central disk and long, flexible arms,

which they use for movement; suspension feeders, predators, or scavengers

Echinoidea sea urchins (feed on seaweed using a jaw-like structure on their underside) and sand dollars—have no arms but have five rows of tube feet; spines used for locomotion and protection

Crinoidea sea lilies and feather starsSea lilies live attached to the substrate by a stalk;

suspension feederFeather stars can crawl using long, flexible arms; suspension

feederHolothuroidea sea cucumbersSea cucumbers lack spine, have a very reduced endoskeleton,

and do not look much like other echinoderms; have five rows of tube feet, some of these are developed as feeding tentacles

Chordates bilaterally symmetrical coelomates with segmented bodies; share many features of embryonic development with echinoderms

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