Ctenophora.—The members of this class are mostly small, peculiar jellyfishes which do not form colonies, and are extremely delicate, being usually perfectly transparent. They swim by means of cilia. They never appear in a polyp condition, but are always medusoid in shape.

[CHAPTER XVIII]

BRANCH ECHINODERMATA: STARFISHES, SEA-URCHINS, SEA-CUCUMBERS

STARFISH (Asterias sp.)

Technical Note.—The species of Asterias are widely distributed on both coasts of the United States and may be procured on almost any rocky shore at low tide. Teachers in inland schools can obtain preserved material from the dealers mentioned on p. 453. Most of the specimens should be placed in alcohol or 4% formalin. If fresh material can be had it is well to place at least one specimen for each student in a 20% solution of nitric acid in water for two or three hours, when all of the calcareous parts will have been dissolved, and after a thorough washing the specimen will be ready for use.

External structure (figs. [18] and [19].)—In a fresh specimen or one which has been preserved in alcohol or formalin note the raying out of parts of the body from a common centre. This is characteristic of the body organization of all Echinoderms, and is known as radial symmetry. The lower surface of the body is called the oral (because the mouth is on this surface), while the upper is called the aboral surface. The central part of the body is called the disk. Note on the aboral surface of the disk a small striated calcareous plate, the madreporite or madreporic plate. In the middle (or very nearly in the middle) of this surface of the disk there is a small pore, the anal opening. The entire aboral surface as well as a greater part of the oral side is thickly studded with the calcareous ossicles of the body-wall. These ossicles support numerous short stout spines arranged in irregular rows. Note that some of the ossicles support certain very small pincer-like processes, the pedicellariæ. In the interspaces between the calcareous plates are soft fringe-like projections of the inner body-lining, the respiratory cæca. Note at the tip of each arm or ray a cluster of small calcareous ossicles and within each cluster a small speck of red pigment, the eye-spot or ocellus.