STARFISH 



197 



227. Nervous System and Behavior.— The nervous system is less 

 highly developed than in the phyla previously studied, there being no 

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

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

 cral 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. 105). The pigment spots are light-perceiving and the 



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Fig. 106. — Bipinnaria larva of Asterias vulgaris. {From Field, in Quar. Jour. Microsc. 

 Sci., vol. 34.) A, ventral view of a larva five weeks old, to show the bilateral symmetry. 

 The body bears a number of lobes, and two bands of cilia, one preoral, the other jjostoral. 

 The latter is not visible where it passes over upon the dorsal surface, but actually is a 

 continuous band. The internal organs are seen through the partly transparent body. 

 X 52. B, lateral view of a three-weeks-old larva, enlarged to the same size as A. X 63. 

 Because of the difference in age there is lack of agreement in certain details, but it is hoped 

 the two views will enable the reader to visualize the larva. 



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 

 contact, light, temperature, and chemicals. 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 substratum, 

 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 restraining these rays, Jennings succeeded in developing in individuals 



