350 



COMPARATIVE ANATOMY 



CHAP. 



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



ffkM-\, cwn. 



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

 plicated masticatory apparatus, i.e. in all Ecldnoidea except the Sj)atan(joida and a 

 few Eokctypoida, 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 ambnlacral 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 

 plates, and those which rise on the interambulaoral 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 Diadeviatoida (Fig. 308, A), 

 an apophysis rises on the peristomal margin 

 of each ambulacral area on each side of the 

 ambulacral suture. The two apophyses of 

 one and the same ambulacrum usually unite 

 at their free ends, which 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, 



T??®,'il'^'^*''"','^?°'"'^-^°^ ™ '^^^' ^^^ ambulacral apophyses, which may 

 unite in pairs to form five auricles. The 

 interambulacral apophyses project less far into 

 the interior of the body. The two apophyses 

 of one and the same interambulacrum together 

 form a ridge w'hich 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 body, 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 apophyses of an 

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

 When the two interambulaoral 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 Clypeastroida, the apophysial ring is interrupted, and consists either of 

 ambulacral or of interambulacral apophyses. 



Fio. 



of a radius and of the two neighbouring 

 interradii of various Eohinoidea. A, 

 Diadematoid. The apophyses of the 

 ambulacral plates {am) form true auriculie 

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

 not by the ambulacral but by the iuter- 

 ambulacral plates, forming what are called 

 false auricles. In C (also a Cidaroid) these 

 interambulacral plates have fused. 



