368 '■Wf ANIMAL KINGDOM 



once it is pressed against a smooth surface it will stick tightly. Suction 

 is improved by a sticky secretion from the end of the foot, and it can 

 be increased further by the contraction of small muscles attached to a 

 connective tissue "spray" that pulls on the middle of the suction cup. 

 To release the foot, longitudinal muscle fibers in the tube contract and 

 lift the edges of the sucker. When these longitudinal fibers contract 

 completely, the tube foot is drawn up close to the body and its fluid is 

 forced into the ampulla. In creeping the tube feet work asynchronously. 

 Each foot elongates in the direction of motion, attaches to the bottom, 

 and then is swung beneath the body so as to propel the body forward. 



A small lateral canal joins each tube foot with a radial canal (Fig. 

 19.8 B). The lateral canal is valved so that it can be closed or opened to 

 adjust the amount of fluid in the tube foot and ampulla. The five radial 

 canals join a circular ring canal in the lower part of the disc, and from 

 this a single stone canal leads upward to the madreporite. All of these 

 cavities together are the water vascular system which is a unique 

 echinoderm feature, derived from a portion of the coelom. 



The tube feet at the tips of the rays are long and slender, acting as 

 tentacles to explore the bottom as the starfish moves. No one or two rays 

 are permanently anterior, but at any moment if a starfish is cut like a 

 pie into five pieces the rays that were anterior will creep with the ray 

 tips forward for a few minutes, whereas the rays that were posterior will 

 creep with their bases forward. Hence, temporary anteroposterior axes 

 are established in the starfish. 



The only sense organs in addition to the tactile tube feet are small 

 eyespots at the tips of the rays (Fig. 19.8 C). Each eyespot is composed 

 of about one hundred pigment cups, each lined with a layer of retinal 

 cells. The starfish, however, shows no evidence of form vision, and has 

 only general movements toward or away from light. The tips of the arms 

 are curved so that the eyespots face outward or upward. 



Scattered throughout the epidermis are numerous cells that act as 

 chemoreceptors. Starfish have been observed to move toward dead fish 

 and are often caught in baited traps such as those used for crabs and 

 lobsters. 



The mouth is in the center of the lower surface surrounded by a 

 membranous area, the peristome. The mouth opens directly into a large 

 cardiac stomach, which in turn opens upward into a smaller pyloric 

 stomach. The digestive tract continues upward as a small intestine that 

 ends at an anus in the middle of the upper surface of the disc. Five 

 large, hollow, branched digestive glands that extend out to the tips of 

 the rays open into the pyloric stomach. The intestine has a lobulated 

 diverticulum, the rectal sac, of unknown function. In feeding, the 

 cardiac stomach everts through the mouth and spreads over the food 

 (Fig. 19.7 A). A copious fluid containing powerful enzymes is secreted by 

 the digestive glancls and poured over the food, rapidly reducing it to a 

 broth. The digested material is then swallowed, and the nutrients are 

 absorbed by the gastrodermis lining the pyloric stomach and digestive 

 glands. Five pairs of retractor muscles (Fig- 19.6 D) from the cardiac 



