CLASS [ 



ECHINOIDEA 



215 



which probably represents five fused basals, and is usually pierced in 

 four or in all of its interradial angles by genital pores (Fig. 345, E, F). 



l'< riproct. This is within the apical system in all regular Sea-urchins, and 

 at a variable distance beyond it in the median line of the posterior inter-, 

 ambulacrum when the test is exocyclic. The plates of the periproctal 

 membrane may be few and symmetrical, or numerous and irregular, and 

 arranged concentrically about the anal orifice. The Saleniidae have a large 

 suranal plate immediately in front of the latter. The periproct is usually 

 circular, but may vary from elliptical to pyriform. Its position on the test 

 is of great systematic importance. 



Peristome. This is more or less central and actinal in the Endocydica, and 

 is decagonal or pentagonal in contour ; and there are not infrequently five 

 or ten incisions along its edges for the reception of external branchiae. 

 In the Exocydica it is variable in shape and position, but is usually actinal, 

 and eccentric in front. Sometimes it is semilunar in outline, with a posterior 



FIG. 34r, 



Dental system of the recent Echinus 

 ' rice, sfon from within the 

 test, a, Jaws or pyramids ; b, Tooth ; 

 c, Epiphysis or arcus ; d, Brace ; 

 e, Rotula or compass ; p, Peristome ; 

 (mi, Ambulacrum ; an, Auricles (after 

 Bert rand). 



Dental apparatus of Sphuer echinus. A, View of the flat upper 

 surface. , Half -pyramid with overlying pieces removed ;,//,<', 

 Epiphyses ; d, Brace (Zwischenkieferstiick) ; e, x, Rotulae or com- 

 passes ; w, Symphyses of the half- pyramids, with alveoli on the 

 inner face. The letters y and z mark the Amb and lAmb areas, 

 respectively. B, C, Half-pyramids seen sidewise and from within, 

 a, Lateral wings ; b, Tooth ; c, Supra-alveolar process. 



labrum. Its plated membrane is attached to the pyramids of the jaws in 

 Gnathostomes ; otherwise the inner edge is free. 



Around the peristome the pairs of ambulacral pores are often crowded. Or 

 they may form a kind of rosette, in which some of the pairs are wider apart 

 than others, and some are put out of place and doubled ; such an arrangement 

 is called a phijllode. In the Cassidulidae the phyllodes are sunken, and the 

 interambulacral areas between them are swollen so as to form lip-shaped ridges 

 (bourrelets buccaux), the whole forming a fioscelle about the mouth. 



Jaws. All Sea-urchins, with the exception of the Spatangoids, and pos- 

 sibly some of the Holectypoids, are provided with a powerful masticatory 

 apparatus, commonly known as "Aristotle's Lantern." In the Endocydica 

 (Figs. 346, 347) the system consists of five hard interradially situated teeth, 

 which are in relation with as many pyramids resting upon the peristome 

 internally, and whose free ends are capable of being moved towards one 

 another or apart. The pyramids form collectively a hollow cone or three- 

 sided pyramid, with the apex directed downwards, and the teeth slightly 

 projecting beyond the peristome. The pyramids are connected with one 

 another by muscles at their sides ; at their upper junction there are five 



