350 



COMPARATIVE ANATOMY 



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



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

 plicated masticatory apparatus, i.e. in all Echinoidea except the Sjiatangoida 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 the 

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

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

 plates next in order also take part in their formation. 



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

 j^lates, 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, which 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 two apoph3'ses of 

 one and the same ambulacrum usually unite 

 at their free ends, which project into the 

 liody, 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, M'hich may 

 unite in pairs to form five auricles. The 

 interambulacral apophyses project less far into 



Fig. 308.— The perignathic apophyses 

 of a radius and of the two neighbouring 

 interradii of various Echinoidea. A, 

 Diadematoid. The aiKiiiliyses of the 



{aur). B, Cidaroid. Apophyses are formed, 

 not by the ambulacral but by the inter- 

 ambulacral plates, forming what are called 

 false auricles. In C (also a Cidaroid) these 

 interambulacral plates have fused. 



ambulacral jdates {nm) form true auriculai the interior of the body. The two apophyses 



of one and the same interambulacrum together 

 form a ridge Avhicli runs along the edge of 

 the peristome, and connects two neighbour- 

 ing auricles ; these ridges are generally fused 

 with one another and with the auricles. 

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

 jects into the liody, may be compared to a circular wall with high arched gateways 

 at five radially arranged points. The five arched gateways would represent the 

 auricles, i.e. the five pairs of ambulacral apophyses, and the circular wall would 

 be formed of the five pairs of interambulacral apophyses. 



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

 ambulacral apophyses are wanting, but the interambulacral apophyses are all the 

 more strongly developed, and form ear-shaped processes. The two ajjojihyses of an 

 interambulacrum are connected by a suture at their bases, but diverge at their tij^s. 

 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 wanting in a few Holectypoida ; where they 

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



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

 ambulacral or of interambulacral apophyses. 



