9 8 



SYSTEMATIC ZOOLOGY 



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iru.ami. 



FlG. 88. Arrangement of skeletal 

 plates at the aboral pole of '.a sea 

 urchin, amb, ambulacral zones ; gen, 

 genital plates; int. amb, interambu- 

 lacral zones ; madr, mad reporic plate; 



pigment spot which they may bear 

 has been considered by some zoolo- 

 gists as an eye (Fig. 88). 



The spines of the sea urchin are 

 attached to the shell bv means of a 

 ball and socket joint, and are pretty 

 freely movable by means of muscles 

 (Fig. 89). If the spines are re- 

 moved, the shell is seen to consist 

 of twenty meridional series of plates, 

 firmly united with one another so 



, ocular plates; . periproct that they arg i mmova bi e , and bear- 



or anal plates. (After Leuckart, from J 



Parker and Hasweii's Manu ing numerous bosses or knoblike 



projections which are the balls on 

 which the spines rest with their concave ends (Fig. 90). 

 These twenty series of plates form ten double bands, five 

 which are pierced by numerous small holes through which the 

 tube feet project ; these five lie in the same radii as the ocular 

 plates, and the plates of which they are composed are called 

 the ambulacral plates; the rive remaining series alternate with 

 these and their plates are called the interambulacral plates, 

 terminating on the aboral side in the genital plates. The chew- 

 ing apparatus is highly developed in the sea 

 urchins. The five teeth already referred to 

 are firmly fixed in sockets composed of cal- 

 careous plates, which are closely united by 

 other plates and by muscles. The whole 

 structure is called Aristotle's lantern (Fie;. 

 91). The mouth opens into an oesophagus, 

 which passes into an elongated intestine, 

 coiled up within the shell, and terminating 

 in the anus. 



The larva of the sea urchins, known 

 technically as the pluteus larva (Fig. 92), is 

 bilaterally symmetrical as in the other Echi- 

 nodermata, but its arms are supported bv a 

 delicate calcareous skeleton, and therefore 

 more rigid than in the starfish larva. The 

 adult develops in very much the same wax- 



Fir;. 89. Diagram of 

 spine ot sea urchin, 

 :ng method of 

 articulation. b, liga- 

 ment ; in, mus 

 ( From Parker and 

 Hasweii's Text-book.) 



