224 Description of the Plates. 



the regular Echinodermata, such as the Asteridae, Ophiuridae; and 

 the regular Echinoidea^ to the ambulacral regions in which all the 

 five rays contribute equally, do not in adult liCe keep, when in loco- 

 motion^ any one of these always pointing anteriorly ; and it is by 

 the relations of the anus in the proctuchous forms and of the 

 madreporic tubercle when the anus is absent, that we are enabled 

 to divide the five rays into two sets of them and two rays respec- 

 tively. In the Ophiuridae, in which there is no anus, and in which 

 the madreporic tubercle is not always visible externally, being fused 

 with one of the interradially-placed circumoral plates, the adult 

 animal may seem to be perfectly radiate, and we have to refer to 

 the pluteiform larva for proof of its essentially bilateral character. 



I. Central radius of trivium; a line drawn along the long axis 



of this ray to the madreporic tubercle g, would, in the undis- 

 turbed condition of the parts, have the anus a little on its 

 left ; and if prolonged, would pass down the interradial space 

 of the bivium IV and V. 



II. Right radius of trivium, with the greater part of the tegu- 



ment left in situ. The inward prolongation of the external 

 ossiferous envelope is well seen in the interradial space be- 

 tween arm I and arm II ; and the generative gland of either 

 ray is seen on either side of the septum thus constituted, 

 with which it is connected by a single efferent duct with a 

 cribriform opening on to the exterior. The dorsal integu- 

 ment contains a large number of ossicula imbedded in its 

 substance, some of which carry small conical prickly spines, 

 whilst others simply connect the spinigerous ossicles into a 

 reticulation. Down the centre of each ray the spinigerous 

 tubercles are in this species arranged with considerable regu- 

 larity, so as to form a mesial series ; in the other portions 

 of the dorsal area, they are scattered irregularly. The in- 

 tervals between the dorsal ossicula are perforated by respi- 

 ratory pores, through which coecal processes of the peri- 

 visceral sac protrude, and are exposed to the circumambient 

 aerating medium. Pedicellariae of considerable size are 

 scattered over the interspinal areae, and smaller and incon- 

 spicuous ones surround many of the spines in a circle. They 



