PHYLUM ECHINODERMATA 



397 



fiercer 



ten plates, the apical 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 bivium, 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, 

 our), 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 (e), the apices of which are to be 



irU.ami. 



Fig. 323. — Apical system of plates and 

 aboral extremities of zones of the shell 

 of a Sea-urchin, atnb. ambulacral 

 zones ; gen. genital plates ; int. amb. 

 inter-ambulacral zones ; vuulr. madre- 

 porite ; oc. ocular plates ; peripr. peri- 

 proct. (After Leuckart.) 



Fit;. BM. — Lantern of Aristotle of Echinus. 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 Invei-tebrates, after MtUler.) 



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. 



