STARFISH 



201 



central ganglion. The system consists of a nerve ring encircling the 

 perioral membrane, radial nerve cords lying at the bottoms of the ambula- 

 cra] grooves and reaching to the tips of the rays, nerves on the dorsal 

 surface of each ray which converge toward the center of the aboral disc, 

 and scattered nerve cells and sense cells lying among the cells of the 

 epidermis and distributed above the nerve cords. The principal sense 

 organs are the pigment spots, one at the tip of each ray, below a so-called 

 tentacle (Fig. 112). The pigment spots are light-perceiving and the 

 tentacles tactile organs. The tube 

 feet and the pedicellariae are very 

 sensitive to touch. 



Starfishes as a rule are not very 

 active in the daytime, but at night 

 they move about in search of food. 

 They respond to such stimuli as con- 

 tact, light, temperature, and chemi- 

 cals. Jennings carried out some 

 experiments which indicated the 

 ability of a starfish to form a habit. 

 When a starfish is placed upon its 

 aboral surface it draws two or three 

 of its rays back under its body, 

 attaches the tube feet to the sub- 

 stratum, and turns itself over. In 

 most starfishes there is a tendency 

 regularly to use certain rays, and 

 these were determined for those 

 experimented upon. By restrauiing 

 these rays, Jennings succeeded in 

 developing in individuals the use of 

 others. One animal was trained in of a group of starfishes noted for their 



180 lessons ten on each of eiffhteen '"^generative ability. A to C, three speci- 

 iOU lessons, len on eacn OI eigmeen ^^^^ showing regeneration of the whole 



days, so to use certain rays; after animal from one ray. in such a case five 



an intprvq] of spvpn davs and whpn arms are regenerated making six altogether, 



an mier\ ai oi seven aays ana wnen g^^ ^^^^^^ ^-^^ normal number. When the 



left to its own initiative it was still disk is uninjured only the rays lost are 



using them. This type of action '^s^''^'^^^^' ^« i" ^- Natural size. 

 acquired as a result of the repetition of an act is known as a habit. 



230. Reproduction. — Both sperm cells and egg cells are set free in 

 the water, where fertilization takes place. The eggs undergo total and 

 equal cleavage, after which follows a typical embryogeny, including the 

 development of a hollow blastula, a gastrula formed by invagination, 

 and a triploblastic embryo. A larva known as a hipinnaria (Fig. 114) 

 is produced which is bilaterally symmetrical but which gradually meta- 

 morphoses into the radially symmetrical adult. 



Fig. 115. — Linckia guildhigii Gray, one 



