262 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
Such, then, is the so-called ambulacral system of 
the Star-fish. Passing over another system of vessels 
which I need not wait to describe (Fig. 36, g, h, D), 
we come next to the nervous system. ‘This 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 Star-fish 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 the 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 dise (Fig. 37). 
Again, in another species the rays have begun to 
branch, these branches again to branch, and so on 
till the whole animal looks like a mat. But the 
most extreme modifications are attained in the 
