220 MARINE INVERTEBRATES 
in others they are hollow, sharp, and brittle; again in others they 
are short and silky, or very long and serrated, and so on. In 
one species they are so broad and flat as to resemble little sails. 
(See Dorocidaris.) (Plate LVI.) 
The mouth, over which five long teeth project, is a part of Aris- 
totle’s lantern, which forms a curious and prominent feature in 
the center of the lower surface. Ten tentacles, like large tube- 
feet without suckers, lie around the mouth. The alimentary 
canal, starting in Aristotle’s lantern, winds in two and a half 
coils around the inside of the shell, supported by mesenteries, 
and terminates in the excretory opening on the aboral surface. 
The sea-urchin has also a water-vascular system similar to that 
of starfishes (page 206). When the tube-feet are distended they 
project beyond the spines, and enable the animal to move slowly 
about; but sea-urchins are less active than starfishes, and 
although they are so well protected by spines and have few 
enemies after maturity, they lie in sluggish idleness in secluded 
places, and for further concealment often cover themselves with 
seaweeds or stones. 
The sea-urchin has a nervous system, which starts in a ring 
around the mouth, ramifies through the body, and terminates in 
the eye-plates. The tube-feet and all the spines are under nervous 
as well as muscular control. Egg-sacs lie under the apex of the 
shell and open by separate ducts into the five plates on the small 
upper disk. Through these the eggs are discharged into the 
water, where they become free-swimming larve, called Pluteus. 
This immature sea-urchin (Pluteus) undergoes several curious 
transformations in the course of its development, and does not in 
any way resemble the mature animal. In spring the apical disk 
of the females will often be found covered with orange-colored 
ova, and that of the males with white sperms. In the growth of 
the animal, in its mature form, the shell enlarges by accretions 
of lime on the individual plates, or ossicles, of the exoskeleton, 
and by new plates formed around the apical disk. Sea-urchins 
are compared to starfishes folded over, the eye-specks on the ends 
of the rays meeting in a small area around the excretory opening, 
the ambulacra following spherical lines and leaving the mouth, as 
before, on the ventral side. 
