350 



COMPARATIVE ANATOMY 



CHAP, 



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



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

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

 few Holcctypoida, 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 ambulacra! or iuter- 

 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 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 Diitdcmatoida (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, 

 Fie, 308. -The perignathic apophyses in all) ten am bulacral apophyses, which may 



of a radius and of the two neighbouring ., ,- c ' , mi 



unite in pairs to form hve auricles. The 

 mterradn of various Echinoidea. A, 



Diadematoid. The apophyses of the interambulacral apophyses project less far into 

 ambulacral plates (nm) form true auricula; the interior of the body. The two apophyses 

 (.</) B, Cidaroid. Apophyses are formed, o f one au( j the same interambiilacriim together 



form a ridge which runs along the edge of 



not by the ambulacral but by the iuter- 

 ambulacral plates, forming what are callnl 

 false auricles, in C (also a Cidaroid) the.se the penstome, and connects two neighbour- 

 interambulacral plates have fused. 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 Cii/ttmii/i/. (Fig. 308, B and C) the apophysial ring is interrupted. The 

 .iinliiilacral apophyses are wanting, but the interambulacral apophyses are all the 

 inure strongly developed, and form ear-shaped processes. The two apophyses of an 

 mterainbulacrum 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 wanting in a few Holcctypoida; 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. 



