272 



THE RISE OF ANIMAL LIFE 



portion of the disc has been removed with 

 it. Experiments have been tried in which 

 all five arms were removed from the disk, 

 and in some cases the disc was able to re- 

 generate the five rays. 



The ossicles of the endoskeleton are 

 joined together by a network of connective 

 tissue and muscle fibers. Lying within the 

 skeleton and extending through all portions 

 of the body is the coelom. It contains the 

 internal organs and a lymphlike fluid which 

 carries free amoebocytes, thus resembling 

 the fluid of the ambulacral system. In cer- 

 tain regions the coelom comes close to the 

 external epidermis which forms a tiny 

 finger-like extension, and in these structures, 

 called dermal branchiae, the respiratory 

 exchange of gases takes place. The amoe- 

 bocytes gather waste materials and escape 

 from the body through these same bran- 

 chiae. 



When feeding, the starfish seizes the vic- 

 tim with its arms and secures its grasp by 

 attaching the tube feet (Fig. 12-24). The 

 sac-like stomach, which consists of a large 

 lower portion, the cardiac stomach, and the 

 smaller upper region, the pyloric stomach, 

 is then everted through the mouth. If the 

 captured animal is small enough, the 

 stomach may completely surround it. Re- 

 tractor muscles in the arms, just below the 

 digestive glands, draw the everted stomach 

 back into the body to complete digestion. 

 If the animal is large it is digested in por- 

 tions while the stomach remains everted. 

 The digestive juices flow from the pyloric 

 region of the stomach and the hepatic caeca 

 (paired digestive glands of each ray) until 

 the remaining food is small enough to be 

 withdrawn into the pyloric stomach. Very 

 often partially digested food enters the 

 hepatic caeca, as well as other portions of 

 the digestive system, and absorption takes 

 place in these various organs. The digested 

 food passes into the coelomic fluid where it 

 is distributed. Attached to the dorsal por- 

 tion of the pyloric stomach is a short in- 

 testine with rudimentary rectal caeca and 



a small anal opening on the aboral surface 

 of the disc. 



The circulatory system of the starfish is 

 reduced to such an extent that it can 

 scarcely be called a circulatory system at 

 all. There are vessels encircling the mouth 

 and extending down into each ray, but they 

 are too inadequate to transfer the digested 

 material to all parts of the body. Instead, 

 the fluid of the coelomic cavity trans- 

 ports the food to various parts of the body. 



The nervous system of the starfish shows 

 the same radial symmetry seen in the other 

 parts of the body and is, in general, simple. 

 It consists of a nerve ring surrounding the 

 mouth, giving off five branches, one to each 

 arm, called radial nerves (Fig. 12-21). Two 

 other systems lie internally, one on the oral 

 side and another near the aboral side. Each 

 part of the nervous system seems to function 

 independently. The starfish has only a few 

 sense organs. An eye and a tentacle are 

 located at the tip of each ray, and the pedi- 

 cellaria function as dermal sense organs. 



Experiments show that the nervous sys- 

 tem of the starfish is sufficiently organized 

 to exhibit definite responses. A hungry star- 

 fish, placed in a pan of sea water containing 

 bits of pulverized mollusk, will move to- 

 ward the food. This is a distinctly positive 

 chemical response. 



Through its eyes the starfish reacts posi- 

 tively to light. This response is best shown 

 by removing the eyes of four rays, allowing 

 one to remain. With but one eye, the star- 

 fish will continue to react positively to light, 

 but if the remaining eye is removed, orien- 

 tation to light is lost. 



Professor H. S. Jennings tried memory ex- 

 periments on starfish to see whether the 

 animal is able to learn. He found that, after 

 subjecting a starfish to 180 lessons over a 

 period of eighteen days, it could be trained 

 to use a particular arm to right itself after 

 it had been turned over on its aboral side. 

 After a lapse of one week, however, only 

 one of the many animals tested remem- 

 bered its training. 



