PHYLUM ECHINODERMATA

This phylum of “spiny skinned” animals is represented by 6,500 extant marine species and 13,000 fossil forms. Known as sea stars, brittle stars, sea urchins, sea cucumbers, sand dollars and sea lilies, echinoderms are characterized by an endoskeleton of calcareous ossicles, bilateral larvae and pentamerous, radially symmetrical adults. Other interesting features found in the phylum include pedicellariae, and a water vascular system employing tube feet for feeding and locomotion. Class: Asteroidea These are the sea stars. Some recent texts combine the sea stars and the brittle stars in a class Stelleroidea, with the sea stars in a subclass Asteroidea and the brittle stars in a subclass Ophiuroidea. The traditional system of classifying these organisms will be used here. Sea stars are star-shaped with arms not sharply set off from the central disc. Furthermore, the length of their arms (or rays) are 1-2 times the diameter of the central disc. Tube feet are usually provided with suckers. In addition to dried sea stars, live specimens of the ocher star, Pisaster ochraceous and the bat star Patiria miniata may be available in lab. External anatomy. The side with tube feet is ventral or oral and the opposite side is dorsal or aboral. Find the central disc and look for the madreporite, a small, off-center, colored plate locate on the aboral side. This is the opening to the water vascular system. The pair of arms closest to the madreporite is the bivium. The other three arms constitute the trivium. On the aboral surface between the madreporite and beginning of the center arm of the trivium lies the inconspicuous anus. Spines cover the disc and arms. Observe a living sea star and note the soft respiratory papillae or dermal branchiae between the spines; these are the respiratory organs. Using a dissecting scope find the pedicellariae, miniature jaw-like structures that grasp foreign objects. Examine the oral surface and find the centrally located mouth surrounded by a membranous peristome and a circle of protective oral spines. The oral side of each arm has an ambulacral groove housing two or four rows of tube feet or podia. Internal anatomy. Examine the models of a sea star and the contents of the arms. Each arm contains a pair of feathery pyloric (hepatic) ceca or hepatic glands. Hepatic ducts connect the pyloric ceca to the pyloric stomach. Ventrally, the pyloric stomach connects to the larger, thin-walled cardiac stomach that ends at the mouth. Dorsally, the pyloric stomach attaches to the intestine, which terminates at the anus. Beneath the pyloric ceca lay the buff colored gonads. Gonoducts from each gonad travel to the aboral surface and exit as ten inconspicuous genital pores. The water vascular system is best seen beginning at the madreporite. Connected to this exterior opening is the vertical stone canal leading to a horizontal ring canal encircling the mouth. Radial canals exit the ring canal and travel to each arm. Numerous bulbous ampullae reside atop the aboral surface of the radial canals and each represents the thin-walled swollen portion of an orally directed tube foot. All portions of the water vascular system are interconnected.

Figure 1. Draw the aboral surface of a sea star showing the disc and one ray. Make the disc 3 cm in diameter. Label all anatomy.

Figure 2. Draw an oral view of a sea star showing the disc and one ray. Make the disc 3 cm in diameter. Label all anatomy. Figure 3. Draw a cross-section of a sea star arm 8 cm across. Label all anatomy.

Class: Ophiuroidea Brittle stars are echinoderms with their arms sharply set off from the central disc. The length of the arms is 5 to 6 times the diameter of the central disc. Tube feet lack suckers. Included in this class are the strange basket stars.

Figure 4. Draw the aboral surface of a brittle star with a disc diameter of 3 cm. Label all anatomy.

Class: Echinoidea This group includes the globular, armless sea urchins, heart urchins and sand dollars. Their ossicles have fused to form a distinctive skeleton called a test. Most have movable spines and tube feet equipped with suckers. Within the mouth lies an interesting chewing mechanism consisting of five teeth called Aristotle’s lantern. Examine the test of a sea urchin and notice the pentamerous radial symmetry. Locate the ambulacral plates with exit holes for tube feet alternating with the interambulacral plates that lack openings.

Figure 5. Draw a section of a sea urchin test 5 cm in diameter. Include at least one ambulacral plate and one interambulactral plate.

Figure 6. Remove a pedicellariae and examine it using a dissection microscope. Make a detailed drawing 3 cm in length.

Class: Holothuroidea Sea cucumbers are echinoderms lacking arms or spines and retaining a reduced skeleton of scattered ossicles imbedded in their flesh. Tube feet equipped with suckers are present and often arranged in five longitudinal rows. Examine members of this class and learn their characteristics. Class: Crinoidea Crinoids or sea lilies attached by an aboral stalk, while feather stars are stalkless, swimming forms. Most species are attached by an aboral stalk during part or all of their lives. In attached forms five arms branch from a cup-like body and bear pinnules or tentacle-like tube feet used for collecting food from the water. Most members of this

class are extinct (about 5000 fossil species) but 600 species are living and inhabit tropical seas. Examine preserved specimens.

Study Questions

Question 1. Although this phylum is pentamerous radially symmetrical, it is actually distinctly bilaterally symmetrical. Explain.

Question 2. Do echinoderms have an exoskeleton or an endoskeleton? Explain.

Question 3. What are the functions of pedicellariae? Question 4. How does a sea star breathe? Question 5. How does a sea star eat and digest food? Question 6. Explain regeneration and autotomy in echinoderms.