350 



COMPARATIVE ANATOMY 



CHAP. 



(0 The Perignathic Apophysial Girdle (Figs. 308, and 348, p. 402). 



In all Echinoidea in which the mouth is armed with five teeth, moved by a com- 

 plicated masticatory apparatus, i.e. in all Eclmwidea except the Spatangoida and a 

 few Holectypoida, processes, directed apically inwards, are found at the peristomal 

 edge of the test ; these serve for the attachment of the muscles and bands of 

 masticatory apparatus. They either consist solely of the ambulacral or inter- 

 ambulacral marginal plates of the peristome bent round inwards, or else a few oi 

 plates next in order also take part in their formation. 



These processes may be divided into those Avhich rise on the ambulacral marginal 

 plates and those which rise on the interambulacral marginal plates. The former 



may be called the ambulacral apophyses, 

 the latter the interambulacral apophyses. 



The apophysial circle is closed or inter- 

 rupted. In the former case, Avhich is best 

 illustrated by the Diadematoida (Fig. 308, A), 

 an apophysis rises on the peristomal margin 

 of each ambulacral area on each side of the 

 ambulacral suture. The tAvo apophyses of 

 one and the same ambulacrum usually unite 

 at their free ends, Avhich project into the 

 body, in such a way as together to form a 

 kind of arch ; this is called an auricle, and 

 affords passage for some of the important 

 organs (for the trunks of the radial ambulacral 

 vessels, of the nerves, etc. ). There are thus, 

 in all, ten ambulacral apophyses, which may 

 unite in pairs to form five auricles. The 

 interambulacral apophyses project less far into 

 The tAvo apophyses 



of one and the same interambulacrum together 

 form a ridge AA'hich runs along the edge of 

 the peristome, and connects two neighbour- 

 ing auricles ; these ridges are generally fused 

 Avith one another and with the auricles. 



Such a closed apophysial ring, which rises on the edge of the peristome and pro- 

 jects into the body, may be compared to a circular Avail with high arched gateAvays 

 at five radially arranged points. The five arched gateAvays Avould represent the 

 auricles, i.e. the five pairs of ambulacral apophyses, and the circular Avail Avould 

 be formed of the five pairs of interambulacral apophyses. 



In the Cidaroida (Fig. 308, B and C) the apophysial ring is interrupted. The 

 ambulacra! apophyses are A\-anting, but the interambulacral apophyses are all the 

 more strongly developed, and form ear-shaped processes. The tAvo apophyses of an 

 interambulacrum are connected by a suture at their bases, but diverge at their tips. 

 When the two interambulacral apophyses standing at the sides of an ambulacrum 

 approximate above it (the ambulacrum), but without fusing, a false auricle may 

 be formed. 



The ambulacral apophyses are also Avanting in a feAv Holectypoida ; Avhere they 

 are present, they do not unite in pairs to form auricles. 



In all Clypeastroida, the apophysial ring is interrupted, and consists either of 

 ambulacral or of interambulacral apophyses. 



FIG. 308. The perignathic apophyses 

 of a radius and of the two neighbouring 

 interradii of various Echinoidea. A, 

 Diadematoid. The apophyses of the 

 ambulacral plates (am) form true auriculas the interior of the body, 

 (cutr). B, Cidaroid. Apophyses are formed, 

 not by the ambulacral but by the inter- 



interambulacral plates have fused. 



