220 MARINE INVERTEBEATES 



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 larvaa, 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. 



