XIV. Echinodermata 



ASTEROIDEA 



The nervous system of the starfish is about the same in all 

 forms which have been studied. Only minor unimportant differ- 

 ences can be recognized and some of these may be due to the differ- 

 ent conditions under which the observations were made or the 

 different methods employed. 



Along the radial and circumoral ambulacral vessels on the 

 oral side is a median thickening of the surface epithelium. This 

 is the chief part of the nervous system, that is the superficial 

 radial and circumoral system. Separated from these portions by 

 connective substance there are in each arm on each side of the 

 middle line the deep radial bands while within the nerve ring 

 about the mouth there are two deep circumoral bands continuous 

 with the two in each arm. 



From the superficial nervous system fibers may be traced 

 directly to the surface layers of the tube-feet. From the inside 

 nerve rings, fibers follow the ambulacral system. The superficial 

 system is merely a thickening of the epidermis in certain regions 

 while the deep system is a thickening of the surface of the ambu- 

 lacral system. Nerve strands from the circumoral rings, proba- 

 bly from the deep rings, run to the stomach and other viscera. 



In addition to the parts of the nervous system just described 

 there is a rather diffuse network of fibers and probably cells, found 

 in the body-wall outside of the muscles. This last has been called 

 the coelomic. 



Sense cells and perhaps something of a nerve plexus seem to 

 be present below the epidermis all over the aboral and lateral parts 

 of the starfish. Just what relationship all these parts of the ner- 

 vous system bear to each other or how they may be distinguished 

 from each other, has never been made entirely clear. 



Almost any portion of the body seems to be sensitive to touch 

 and there may be other sensations without special organs for their 

 perception. At the tip Of each arm a little tentacle or papilla 

 marks the end of the radial canal and the superficial nerve cord. 

 This little organ has a special epithelium and may be a special 

 organ of touch but Eimer, 1880, considers it as an organ of taste. 



The eye-spot is the most marked sense organ of the starfish. 

 Each arm has. very near the termination of the radial nerve at the 

 tip of the arm, a bright red spot of pigment. A little closer exam- 

 ination of one of these spots shows it to be composed of a number 

 of distinct regions of color. In section these little areas are seen 

 to be little follicles lined with epithelial cells. The cells which line 

 the follicles are spoken of as the visual cells. These are clear at 

 their inner margins but pigmented farther down. Their inner 

 processes come into relation with the nerve strands at the bases of 



