262 JELLY-FISH, STAR-FISH, AND SEA URCHINS. 



Such, tlien, is tlie so-called ambulacral system of 

 the Star-fish. Passing over anotlicr system of vessels 

 which I need not wait to describe (Fig. 06, g, h, I), 

 we come next to the nervous system. Tliis is dis- 

 posed on a very simple plan. It consists of a penta- 

 gonal ring surrounding the mouth, from which a 

 nerve-trunk passes into each of the five rays, to 

 run along the ambulacral groove as far as the ex- 

 treme tip of the ray, where it ends in a small red 

 pigmented spot, about which I shall have more to 

 say presently. Each of these five radial nerves 

 gives off in its course a number of delicate branches 

 to the tube-feet. 



Modifications of the Starfish Type. 



So much, then, for the structure of the common 

 Star-fish. I must next say a few words on the re- 

 markable modifications which this structure under- 

 goes in different members of the Star-fish group. 



In some species tho size of the central disc is 

 increased so as to fill up the interspaces between 

 the rays, the whole animal being thus converted 

 into the form of a pentagon. In other species, 

 again, the reverse process has taken place, the rays 

 having become relatively longer, and being at the 

 same time very active; they look like five little 

 snakes joined together by a circular disc (Fig. 37). 

 Again, in another species the ra3\s have begun to 

 branch, these branches again to branch, and so on 

 till the whole animal looks like a mat. But the 

 mo.^t exticiue modifications are attained in the 



