ix PHYLUM ECHINODERMATA 389 



consisting of two rows, running in a meridional direction from the 

 edge of the peristome to the neighbourhood of the periproct. Of 

 the zones of plates there are two sets, each consisting of five, the 

 members of which alternate with one another. In the case of one 

 of these sets of zones the ambulacral zones or ambulacral areas 

 already referred to each of the plates is perforated towards 

 its outer end by two minute pores, the ambulacral pores, for the 

 protrusion of the tube-feet. In the other five zones, the inter- 

 ambulacral zones or areas, the plates are not perforated. At its 

 a^al_^ndjeach area r .ambulacral or inter-ambulacial T ,_ends-_iiL_a. 

 single ttpic^lrAsite, so that the periproct is surrounded by a ring of 

 ten plates, ^^a^ical system--0i~ plates (Fig. 328). Of these, the 

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

 termed the \ ocular pieties r (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-ainbulacral 

 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 distin- 

 guishes it from the others : this is 

 the madreporite, through which, as in 



the Case of the Structure of the Same FIG 328. Apical system of plates-and 



aboral extremities of zones of the shell 



name in the Starfishes, the madreponc of a sea-urchin, amb. ambulacral 

 canal communicates with the exterior. SS^JSUS^^^ilt 

 The two ambulacral areas between JggjJt "(AtASaSti) perip> '' 

 which the madreporite lies constitute 

 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. 330, 

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

 lies a complex structure termed Aristotle's lantern (Fig. 329). 

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

 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. 

 Firmly united to the base of the alveolus is a stout bar, the 

 epiphysis (6). Adjacent epiphyses are in close contact with one 

 another, and running inwards from their points of union are five 

 radially-directed, stout bars, the rotulce (c), the inner ends of which 

 unite to bound a circular aperture through which the oesophagus 

 passes. With the inner end of each rotula is movably articulated 



