METAMORPHOSIS 53 



fish. On the inside of the floor of each ray are the ampulla, 

 small bulb-like water-sacs, which are connected with the tube- 

 feet on the outside of the ray. " By a contraction of the deli- 

 cate muscles in the walls of the ampullae the fluid in the cavity 

 is compressed, thereby forcing the tube-feet out. By the con- 

 traction of muscles in the tube-feet they are again shortened, 

 while the small disk-like terminal sucker clings to some firm 

 object. In this way the animal pulls itself along by successive 

 steps." By the aid of these ambulacral or tube-feet the starfish 

 is able to turn over if placed upon its back. They also act as 

 suckers to fasten the starfish to the rocks. When once this is 

 accomplished, arm after arm may be broken off before the 

 animal can be pulled loose or the feet will relax their hold. 



So-called blood canals accompany the ring and radial canals, 

 and associated with them are sometimes two intestinal blood- 

 vessels. 1 



Nervous System. " There is a nerve ring and radial nerve, 

 frequently in the ectoderm, to which may be added an entero- 

 ccelic or apical nervous system, possibly of peritoneal origin." 



The circulating fluid is somewhat lymph-like and the circula- 

 tion slow. 



" Respiratory organs are represented by the branchice, or thin- 

 walled outpushings of the ccelom, either around the mouth, as in 

 the Echinoi'dea, or on the aboral surface, as in the Asteroi'dea, 

 the bur SOB of the Ophiuroi'dea, the branchial trees of the Holothu- 

 roi'dea, and the various parts of the ambulacral system." 2 



The alimentary tube is complete, that is, shut off from the 

 body cavity and runs through the body. Its length depends upon 

 the food of the echinoderm. In carnivorous forms, as the star- 

 fish, it is short, but in vegetable feeders, as the sea-urchins 

 and sea-cucumbers, the alimentary tube is two or three times 

 the length of the body. 



Multiplication is sexual, as a rule, the sexes being separate 

 except in rare cases. Fertilization takes place in the water. 

 They never form colonies by budding. 



The metamorphosis, or change from the larval to the adult 

 form, is as marked as that from the caterpillar to the butterfly. 



1 Hertwig's " Manual of Zoology," Kingsley, p. 331. 



2 Ibid. 



