11 echinodermata

Phylum Echinodermata

General Characteristics• Adults exhibit pentamerous radial symmetry• Radially symmetry is secondary; larvae are bilaterally symmetrical andundergo metamorphosis to become radially symmetrical adults. Echinoderm larva

General Characteristics cont.•Poorly ganglionated; possess few sensory structures• Body wall contains an endoskeleton of calcareous plates - ossicles

General Characteristics cont.• Possess a network of canals throughout the body - water vascularsystem.• The canals are connected to extensions called tube feet (=podia),located on the oral surface• The water vascular system is important for locomotion, feeding, andgas exchange.• Sexes are separate; gametes shed into the water; fertilization is external

Echinoderm Diversity

Class Asteroidea• Typically have 5 arms which merge with a central disc• Mouth is located in the center of oral surface which is directeddownward

Water Vascular System• On the aboral surface is the opening of the water vascular system themadreporite (=sieve plate)• Water enters the madreporite and goes through the stone canal canalto the ring canal• Water then passes through a radial canal extending into each arm• All along the length of these canals are lateral canals that terminatein a bulb-like structures called ampullae equipped with tube feet• Tube feet line the grooves on the oral surface - ambulacral grooves

How the Podia Operate• Ampulla contract and force fluid into the podia causing it tobecome extended• Suckers at the tips of the podia come into contact with the substrateand adhere to the surface• Then the podia contract, thereby forcing water back into theampulla, and the body is pulled forward

Nutrition• Mouth leads to a 2-partstomach: a large cardiacstomach and a smallerpyloric stomach• The pyloric stomachconnects with digestiveglands (=pyloric cecae) thatruns into each arm• A short intestine extends from from the pyloric stomach to an anus onthe aboral surface• Associated with the intestine are rectal cecae that pump the fecalwastes out of the anus

Additional Characteristics• The endoskelton is made up of calcareous plates that often penetratethe dermis as spines• Between the spines and plates are projections called papulae, whichfunction in gas exchange and excretion• Other projections on the body wall include tiny jaw-like appendagescalled pedicellaria

Class Echinoidea• Lack arms• Body is enclosed in a shell or test• Body surface is usually covered with moveable spines

Sea Urchins• Spherical body• Ambulacral plates bearing tubefeet that radiate out toward theaboral surface• Use podia and spines duringlocomotion• The spines are moveable andarticulate with the with thecalcareous ossicles

• Sea urchins generally feed by scraping algae off of rocks• Accomplished via a complex chewing apparatus calledAristotles lantern

Class Holothuroidea• Lack arms• Oral-aboral axis is greatly extended• Endoskeleton is reduced to a few ossiclesscattered over the surface of the animalmaking them rather soft bodied• Some species crawl along the substrateusing podia; others have peristalticlocomotion via muscle contractions Dermal ossicles

• At the oral end of the body are a group of tentacles (modifiedpodia) that surround the mouth; used in feeding• Have a muscular cloaca that is partly used in gas exchange• The actual gas exchange structures are branching structures calledrespiratory trees

Class Crinoidea• Most primitive of the echinoderms• Unusual in that the oral surface isdirected upward• Aboral surface is attached to thesubstrate by means of a bendablestalk• The portion of the crinoid bodyattached to the stalk is called thecrown; bears a number of arms• Along the length of the arms arebranches called pinnules• The arms and the pinnules haveambulacral grooves with suckerlesspodia (secrete mucus)• The ambulacral grooves are heavilyciliated and the cilia is used to directfood to the mouth (=filter feeding)

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11 echinodermata

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