PHYLUM ECHINODERMATA 



;-!97 



ten plates, the npivnl system of plates (Fig. 323). Of these, the 

 five that are situated at the ends of the ambulacral areas are 

 termed the ocular plates {oc), owing 

 to the fact that each of them bears „^ „™« 



a structure once supposed to be a 

 rudimentary eye, but now known 

 to be a tentacle ; while the five 

 opposite the inter-ambulacral areas 

 are termed the genital plates {gen), 

 each of them being perforated by 

 an opening which is the aperture 

 of one of the five genital ducts — ■ 

 the ducts of the ovaries or testes 

 as the case may be. One of these 

 genital plates (madr) has a swollen 

 and spongy appearance, which dis- 

 tinguishes it from the others : this 

 is the madreporite, through which, 

 as in the case of the structure of the same name in the Star- 

 fishes, the madreporic canal communicates with the exterior. The 

 two ambulacral areas between which the madreporite lies con- 

 stitute the bitimn, the remaining three the trivium. 



On the inner surface of the shell, close to the edge of the peri- 

 stome, there project inwards five processes, the auricles (Fig. 325, 

 nur), one opposite each ambulacral area. Within the ring of auricles 

 lies a complex structure termed Aristotle's lantern (Fig. 324). 

 This consists of the five teeth (c), the apices of which are to be 



J. 323. — Apical system of plates and 

 aboral extremities of zones of the shell 

 of a Sea-urcbin. amb. ambulacral 

 zones ; gen. genital plates ; int. amb. 

 inter-ambulacral zones ; iiiadr. madre- 

 porite ; oc. ocular plates ; 'p&'i'jyf. peri- 

 proct. (After Leuckart.) 



Flc. 324. — Lantern of Aristotle of IScllillus. A, two of the five chief component parts apposed 

 and viewed laterally. B, lateral, and C internal view of a single part. a. alveolus ; a', suture 

 with its fellow ; 6. epiphysis ; 6'. suture with alveolus ; c. rotula ; d. radius ; e. tooth. (From 

 Huxley's favei-tebrates, after Mtiller.) 



seen projecting through the mouth, together with a system of 

 ossicles. The teeth are long, curved, and pointed : proximally each 

 is supported by and partly embedded in a pyramidal ossicle, the 

 alveolus (a), consisting of two halves united by a longitudinal suture. 



