SEA-URCHINS. 



659 



Fig. 7. A REGULAR 

 SHA - URCHIN (Cidaris 

 papillata). One-half 

 natural size. 



ECHINOIDEA (Sea-urchins). 



The sea-urchins are the best known and most numerous of echinoderms. 

 They are divided into two groups regular and irregular sea-urchins. Fig. 7 

 shows the test of a regular sea-urchin, Cidaris papillata, 

 which is called the Piper from the supposed resemblance 

 of its spines to the drones of a bagpipe. In the figure 

 most of the larger spines have been removed, but three 

 or four have been left on (S). Several of the plates of 

 the test are seen to bear a rounded tubercle (T), and it 

 is to these that the large spines are attached by a ball- 

 and-socket joint, which is surrounded by muscles that 

 can move the spines in any direction. The bases of the 

 large spines are protected by smaller spines (s 1 ) see figure 

 which are similarly attached to smaller tubercles. The 

 spine-bearing tubercles do not cover the whole test, but 

 are disposed chiefly in five broad zones, interambulacra 

 (I A), which extend from one pole to the other. Alter- 

 nating with these are the narrower ambulacra (A). In 

 these can be seen the holes through which the tube-feet 

 pass. At the summit of the test, where these zones 

 converge, is a star-shaped space covered with mem- 

 brane, and in the middle of this is the anus. Regularly 

 placed round this, at the summit of the interambulacral 

 zones, are five conspicuous plates, each pierced by a 

 hole (G), which is the duct of the underlying generative gland. One of the 

 plates (M) is also pierced by a number of small water-pores, and is the 

 madreporite. Outside these five plates, end alternating with them, are five 

 other plates, each at the top of an ambulacrum, and each pierced by the 

 unpaired tentacles which terminate the water-canals, and represent the 

 unpaired tentacles near the eye at the end of the star-fish arms. These 

 plates, marked O, are therefore called oculars ; in a few cases they do actu- 

 ally bear eyes. The mouth is at the other pole of the body, and is therefore 

 not shown in the figure. It is surrounded by an elaborate arrangement of 

 teeth, found in all regular urchins, and com- 

 pared by Aristotle to a lantern. The gut, 

 which passes from the mouth, coils round 

 inside the test. Such a sea-urchin as Cidaris, 

 or the common egg-urchin, Echinus, shows 

 most clearly the five-rayed symmetry that 

 has been impressed upon the echinoderms. 

 In the irregular sea - urchins, however, 

 one of which, Echinocardium cordatum, 

 is shown in Fig. 8, the animal has become 

 elongated along one of the axes so as to have 

 a superficial two-sided symmetry. This is 

 connected with constant movement in one 

 direction, and such modification is found 

 among those urchins that live on muddy 

 bottoms, and especially in those from con- 

 siderable depths. Not only is the test elon- 

 gated, but the mouth moves forward to the 



AI 



Fig. 8. IRREGULAR SEA - URCHIN 

 (Echinocardium cordatum). Two- 

 thirds natural size. 



