CLASS I ECHINOIDEA Dil 
plate, which probably represents five fused basals, and is usually pierced in 
four or in all of its interradial angles by genital pores (Fig. 345, H, /). 
Periproct.—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 Endocyclica, 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 Ezocyclica it is variable in shape and position, but is usually actinal, 
and eccentric in front. Sometimes it is semilunar in outline, with a posterior 

Fic. 346 ; Fic. 347. 
Dental system of the recent Echinus Dental apparatus of Sphaerechinus. A, View of the flat upper 
miliaris, Leske, seen from within the surface. a, Half-pyramid with overlying pieces removed; ), c, 
test. a, Jaws or pyramids; b, Tooth; Epiphyses ; d, Brace (Zwischenkieferstick) ; e, x, Rotulae or com- 
c, Epiphysis or arcus; d, Brace; passes ; w, Symphyses of the half-pyramids, with alveoli on the 
e, Rotula or compass; p, Peristome ; inner face. The letters yand z mark the Amb and [Amb areas, 
am, Ambulaerum ; aw, Auricles (after respectively. B, C, Half-pyramids seen sidewise and from within. 
Bertrand), a, Lateral wings ; 0, 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 phyllode. 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 floscelle 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 Hndocyclica 
(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 
