oS SVEN LOVÉX, ECHINOLOGIGA. 



tions in Cidaris. The basilar expansion is seen to-bend romid 

 them, and lience tlie suture takes a flexuous course, not the 

 same in different specimens. 



In the Tripnenstes esculentns IjESKE, when an anricle is 

 viewed sidewise from the interradium, flg. 1; from before, that 

 is from the stoma: fig. 2; and from behind, that is from the 

 eavity of the test: fi(j. 3; or the base of the auricle and the 

 ambulacrum in separation: fig. 4, the sntiTre a to c marks the 

 extent of tlie adhesion of the anrienlar expansion to the am- 

 bulacrnm. From a to a' and from a' to h it is the continua- 

 tion of the main snture between the ambulacrnm and the in- 

 terradinm, and the triangulär space included between a, a', 

 and b, fig. 1 and 4, is the even snrfaee of contiguity between 

 them, the auricle projecting into a heel meeting the ridge of 

 the interradium. At b they separate, to form, each by itself, 

 one side of the branchial incisure. From b, fig. 1, 2, S, 4, 

 the course of the suture is on tlio ambulacrum, traversing its 

 plates, and entering their interstices. and then it bends inwards, 

 '', crosses the plates and reaches at a the main suture again. 

 The ambulacral vessel and nerve on their way from the peri- 

 stome take their course över the expansion which receives 

 their branches and becomes penetrated by perforations conti- 

 nuous with the pedicellar pores beneath, fig. 2, ,9, 4. All thi.s 

 is readily seen when the auricle is carefully loosened from the 

 ambulacrum, fig. 4, and its mode of fixation brought into view. 

 The ambulacral plates, the impressions of wliich are seen on 

 the underside of its basilar expansion, are raised so as to form 

 a diagonal crest and on the outer side of tliis a triangulär 

 depression, in which, as in a shallow socket, the auricle is 

 lodged with the part a, c, /;. The surface a, a\ b of the heel 

 is in contact with the interradial ridge. The whole constitutes 

 a fixed articnlation, no d()ul)t with a soft substance intervening. 

 admitting of no appreciable movement, »squamosC' towards the 

 ambulacrum, »harnionic' towards the interradium. Whereas 

 the auricles of the C^idaris were seen to arise solely from the 

 interradia as continuations of their tissues, and while being 

 reabsorbed below to be reconstrued from above, the auricles 

 of the Ectobranchiates are seen to adhere to the ambulacra 

 as distinct and independent parts joined mainly to them by 

 means of articnlation, and to increase by their own growth. 

 both snrfaces, the auricular and the ambulacral, dnring the 



