This laboratory exercise and the next are an introduction to the Phylum Echinodermata and their nearest relatives, the Phylum Chordata. Fish, amphibians, reptiles, birds, and mammals are familiar chordates. Echinoderms and chordates share unique features of embryonic development that set them apart from the rest of the Animalia; these are described below.

The word echinoderm means spiny skin. These are marine organisms with five-fold radial symmetry, commonly called starfish (Class Asteroidea), sand dollars and sea urchins (Class Echinoidea), sea cucumbers (Class Holothuroidea), and sea lilies (Class Crinoidea). They are coelomate. Just under the skin, they have an endoskeleton that is made of calcium carbonate, the substance of corals and clam shells. The skeleton arises from mesodermal tissue, an origin unlike that of bivalve shells, which are secreted by the epidermal sheet of mantle tissue. Sexes are separate in echinoderms and fertilization is external; there is a larval stage with bilateral symmetry. A unique derived feature of the group is its water-vascular system; seawater is pumped through a series of ducts to work the tube feet and the suction cups at their tips.

Class Asteroidea

The Asteroidea are stars or starfish. On the dorsal side you can see the central disk and radiating arms. The central disk has a small sieve plate or madreporite (off center) that connects the water vascular system with the external ocean water. Small fleshy extensions among the spines are the soft, hollow skin gills for respiration; they communicate with the coelom. On the ventral side of the arms there are ambulacral grooves that are filled with the fleshy tube feet. Tube feet and skin gills are both used for gas exchange and excretion of nitrogenous waste. The mouth is in the center of the ventral side. Echinoderms move by alternating the suction and release of tube feet. Asteroids can regenerate lost or damaged arms, if enough of the central disc is intact. An isolated arm soon dies (there is an exception, known in one genus where an arm can regenerate the rest).

Dissection: Remove the skin and skeletal plates from the dorsal side of one arm and of the central disk; leave the madreporite in place. The organs fill the coelomic space. The stomach occupies much of the central disk, and a pair of many chambered digestive glands fill the arm. In feeding, most sea stars evert their stomachs over the food source, secrete enzymes, and suck in the digested food. The stomach is so thin walled, it can be slipped between the closed valves of large prey such as clams. Gonads are located in the base of each arm ventral to the digestive glands.

Echinoderms and chordates share important derived features in the embryos. Both are called deuterostomes (other mouth): the pore leading into the forming digestive system in the gastrula becomes the anus and not the mouth as in invertebrates (protostomes = first mouth); the mouth is another opening (deutero-stome) that forms later in echinoderms and chordates. The zygote (fertilized egg) of echinoderms and chordates undergoes mitosis but the new cells are not yet committed to forming a particular tissue; each, if separated from the others can form an entire individual. This is called indeterminate cleavage and can produce identical twins. In the invertebrates cleavage is determinate and the first two cells are already committed to forming specific parts of the organism; identical twins do not occur.

Click here on starfish development if the subject is included in your lab exercise.

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