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Lab 11a: Phylum Echinodermata, Part I

11 November 1999

Click here for Echinoderms part II

Phylum Echinodermata (= spiny skin) (Wallace & Taylor pp. 271-297)

Echinoderms are an entirely marine phylum, with most species having a benthic lifestyle. Echinoderms are deuterostome coelomates and typically exhibit a basic 5-point radial symmetry as adults (larval forms may be bilaterally symmetrical). Technically, this is callled a pentamerous radial symmetry along an oral-aboral axis. Adults have an internal calcareous skeleton. The water vascular system (WVS), a series of fluid-filled canals derived from the coelom, and that functions in locomotion, is unique to echinoderms. Echinoderms lack cephalization, a heart and any specialized excretory organs. Locomotion is accomplished by the action of tube feet (podia) located within the ambulacral grooves. Respiration and excretion occur by diffusion across the tube feet and papulae. Most species are dioecious (sexes are separate, but hard to distinguish), and have external fertilization externally. There is a free-swimming, bilaterally symmetrical larval stage, although a few species have no larval forms. Most species have the ability to regenerate parts.

Class Asteroidea (star-like) Sea stars (starfish). The basic body plan incorporates five arms radiating from a central disc. Asteroids are slow moving and prey on large invertebrates (such as clams) and small vertebrates. Some authorities group asteroids and ophiuroids as subclasses within a class called Stelleroidea.

Examine live Asterias sp. Using a dissecting scope, observe the tube feet.

Observe external anatomy. Identify oral surface, aboral surface, tube feet (podia), madreporite, anus, ambulacral grooves, arms (rays), spines, papulae (dermal branchiae), pedicellariae, mouth, eyespot, tentacle.

For internal anatomy (procedure outlined in W&T (pp. 273-280). Identify coelomic cavity, cardiac stomach, pyloric stomach, pyloric cecae (hepatic cacae), stone canal, radial canals, Tiedemann bodies, ampullae of tube feet, gonad, ossicles, ambulacral ridge.

Examine slides on demonstration of sea star ray (arm) in cross section and sea star development series, particularly bipinnaria larva and brachiolaria larva

Class Ophiuroidea (snake-like) Brittle stars and serpent stars, the largest group of echinoderms. Like asteroids, ophiuroids have five arms radiating from a central disc, however ophiuroid arms are very long and jointed, and the central disc is much smaller than that of an asteroid. Most ophiuroids do not have ampullae, their arms lack ambulacral grooves, and the coelomic cavity is reduced. Ophiuroids lack an anus, a feature that differentiates them from all other echinoderms. Bursae, invaginations on the oral surface of the arms, function in gas exchange, excretion, and in some species serve as brood chambers for developing embryos.

Examine ophiuroids on demonstration. Identify oral surface, aboral surface, mouth, bursae. Note absence of anus and ambulacral grooves.

Class Crinoidea (lily-like) Sea lilies and feather stars, the oldest group of echinoderms. The crinoid body consists of a cup-shaped calyx containing the digestive system. The calyx is covered by a membrane (tegumen) that contains the mouth and anus. Arms extend upward from the calyx and are composed of a series of jointed ossicles that allow the arms to bend. Tube feet and pinnules, both used in feeding, are located on the arms. Crinoids are the only echinoderms that do not use tube feet for locomotion. Stalked crinoids (sea lilies) are permanently anchored to the substrate by a stalk contructed of a series of calcareous discs. Feather stars (crinoids w/o stalks) use their arms for walking and swimming, and have jointed appendages (cirri) that grasp the substrate when the animal rests.

Examine crinoids on demonstration. Identify pinnules, calyx, cirri, stalk, tegumen, oral surface.

Systematic Classification

Phlum Echinodermata

Subphylum Pelmetazoa ("palm-like") sea lilies, feather stars, cystoids (extinct)

Class Crinoidea sea lillies and feather stars

Feather star

Sea lily (fossil)

Subphlum Eleutherozoa ("truly free") tube ft in 4 rows (vs. 2 in others), pedicullariae

Class Asteroidea sea stars, >=5 arms

Order Paxillosida paxilla, no suckers, tube feet

Ctenodiscus sp.

(Paxillids incl. former Order Valvatida, which do have suckers)

Archaster americanus (NJ native)

Goneaster sp.

Oreaster sp.

Linckia sp.

Order Spinulosida 5 to 18 arms, tube feet with suckers, no pedicullariae

Henricia sanguinolenta blood star, bright red, mults. of 5 arms

Crossaster sp. sun star

Order Forcipulatida 5 to 50 arms, tube feet with suckers, pedicullariae (~forceps!)

Asterias sp.

Asterias forbesi

Coscinasterias sp.

Pisaster sp.

Class Concentricycloidea sea daisies, marginal spines, no arms, rare (no specimens)

Class Ophiuroidea brittle and basket stars, 5 arms, central disc, no suckers, no anus

Order Ophiurida brittle stars, snaky tails

Ophioderma sp.

Ophioderma longicauda

Ophioneris sp.

Order Phyrnophiurida thick skin

Astrophyton sp. sea basket; suspension feeder