THEIR ALLIES. 10 
body. Soon this sac-like body grows longer, and con- 
tracts at intervals ; the intervening parts become unequally 
enlarged, some segments or rings, formed by the contrac- 
tion of the body-walls, greatly exceeding in size those 
next to them; and it thus assumes the appearance of a 
being, more or less equally ringed, such as in the young 
Terebella, here figured, where the ciliæ are restricted to 
a single ring surrounding the body. Gradually the cilix 
disappear and regular locomotive organs, consisting of mi- 
nute paddles, grow out from the side; feelers (antenne), 
jaws, and eyes (simple rudimentary eyes) appear on the 
few front rings of the body, which are grouped by them- 
selves into a sort of head, though it is difficult in a large 
proportion of the lower worms, for unskilled observers to 
distinguish the head from the tail. In the embryo of 
the Crustacean, such as the Fresh-water Crawfish, as 
shown by the German naturalist Rathke ; and also in the 
earliest stages of the Insect, the body at once assumes a 
worm-like form, thus beginning its embryonic life from 
the goal reached by the adult worm. - : 
Thus we see throughout the growth of the worm, no 
attempt at subdividing the body into regions, each endow- 
ed with its peculiar functions ; but only a more perfect 
but all becoming respectively more complicated. For 
€ e, in the fresh-water Nais, each ring is plainly 
Q 
S 
tinguished into an upper and under side, and in addi- 
tion to these a well marked side-area, to which, 2s in the 
marine worm, - ereis, oar and paddle-like organs are 
attached ; in most other worms eye-spots appear on the 
front rings, and slender tentacles grow out, and a pair of 
nerve-knots ( ganglia) are apportioned to each Tings 
Thus, in the Worm the vital force is very equally distrib- 
