Fig. 18.—Dissection of a starfish (Asterias sp.).

Make a drawing of the aboral surface showing all these parts.

On the oral surface note the centrally-located mouth, the ambulacral grooves, one running longitudinally along each ray, and in each groove two double rows of soft tubular bodies with sucker-like tips. These are called the tube-feet and are organs of locomotion. Make a drawing of the oral surface.

Internal structure (figs. [18] and [19]).—Technical Note.—Take a specimen which has been immersed for some time in the nitric acid solution, and with a strong pair of scissors, or better, bone-cutters, cut away all the aboral wall of the disk except that immediately around the madreporite and the anus. Now begin at the tip of each ray and cut away the aboral wall of each, leaving, however, a single arm intact. When the roof of each arm has been carefully dissected away the specimen should appear as in fig. [18].

Note the large alimentary canal, which is divided into several regions. Note the short œsophagus leading from the mouth on the oral surface directly into a large membranous pouch, the cardiac portion of the stomach. By a short constriction the cardiac portion is separated from the part which lies just above, i.e., the pyloric portion of the stomach. From the pyloric portion large, pointed, paired glandular appendages extend into each ray. These are the pyloric cæca. Their function is digestive, and oftentimes they are spoken of as the digestive glands or "livers." The pyloric cæca, as well as the cardiac portion of the stomach, are held in place by paired muscles which extend into each arm. Note two sets of these muscles, one set for thrusting the cardiac portion of the stomach out through the mouth and another for pulling it back, the protractor muscles and retractor muscles, respectively. The starfish obtains its food by enclosing it in its everted stomach and then withdrawing stomach and food into the body. Note that the pyloric portion of the stomach opens above into a short intestine terminating in the anus, and observe that there is attached to the intestine a convoluted many-branched tube, the intestinal cæcum.

Carefully remove a pair of pyloric cæca from one of the rays and note the short duct which connects them with the pyloric chamber of the stomach. Note in the angle of each two adjoining rays paired glandular masses which empty by a common duct on the aboral surface. These glands are the reproductive organs. Note the small bulb-like bladders extending in two double rows on the floor of each ray. These are the water-sacs or ampullæ, and each one is connected directly with one of the locomotor organs, the tube-feet.

Make a drawing of the organs in the dissection which have so far been studied.

Technical Note.—For a careful study of the locomotor organs a fresh starfish should be injected. This can usually be accomplished by cutting one ray off squarely, and inserting the needle of a hypodermic syringe (which has been previously filled with a watery solution of carmine or Berlin blue), into the end of the radial water-tube which runs along the floor of the ray. By injecting here, the whole system of vessels, tube-feet, and ampullæ are filled.

Note a ring-shaped canal which passes around the alimentary canal near the mouth from which radial vessels run out beneath the floor of each ray and from which a hard tube extends to the madreporite. This hard tube is the stone canal, so called because its walls contain a series of calcareous rings, while the circular tube is the ring canal or circum-oral water-ring from which radiate the radial canals. In some species of starfish there are bladder-like reservoirs, Polian vesicles, which extend interradially from the ring canal.