Prof. J. Miiller on the Structure of the Echinoderms. 15 



below the semita contains simple locomotive feet ; that above the 

 semita, on the other hand, as far as the dorsal extremity, presents 

 a sudden transition to feet provided with large suctorial discs, 

 which are strengthened by radiating osseous plates. In the 

 northern Schizaster fragilis these feet are disposed in simple 

 series on each side of the ambulacrum ; in Schizaster canaliferus, 

 however, they constitute a dense series on each side. The lower 

 portion of the anterior radius, from the oral tentacles as far as 

 the semita, contains in Schizaster only a small number (three) of 

 simple locomotive feet at considerable distances from one another 

 upon each side. 



In Amphidetus the peripetalous semita is absent; the four 

 conjugate ambulacra possess branchial feet in their dorsal por- 

 tion, as is commonly the case among the Spatangida. The 

 internal dorsal semita which occurs in this genus divides the 

 azygos radius into an anterior and a posterior part, and, enclosing 

 the posterior part and the apex, it forms an area in which the 

 azygos radius contains only large feet with stellate digitated 

 extremities. On the contraiy, the portion of the radius in front 

 of the semita has simple locomotive feet without digitations. 

 The digitations are supported by calcareous plates. The semita 

 not only cuts off a portion of the anterior, but also a part of the 

 four lateral ambulacra; the portions of the conjugate ambulacra 

 lying within the semita contain no branchial feet, but exceed- 

 ingly small and readily overlooked cylindrical feelers, simple and 

 rounded off at their extremities. The penicillate cii-rated feet 

 around the mouth in the five ambulacra present similar relations 

 in all the Spatangidce which I have examined, and all likewise 

 possess subanal cirri. Brissopis has upon each side a series of 

 six feet with cirri. In Schizaster {canalif erics) , where the subanal 

 semita is absent, and a posterior semita branches off from the 

 peripetalous semita and forms an arch under the anus, seven 

 cirrated feet exist upon each side in a longitudinal series, at a 

 considerable distance from the anus, in the posterior part of the 

 ventral region of the shell ; not between the posterior semita and 

 the anus, but in front of the posterior semita. The stems of 

 the different feet, besides, contain transverse notched calcareous 

 bands in their walls, which are absent in the branchial feet. 



The Clypeasteridx present the greatest deviations in the for- 

 mation of their ambulacra. The feet of the double pores of the 

 petaloid ambiilacra are gill-like ; they are broad, depressed, 

 lobulated, hollow projections between every pair of pores, and 

 are distended by the large ampullar of the ambulacra. The 

 locomotive feet, on the other hand, are cylindrical and exceed- 

 ingly delicate, so that in the Clypeaster they have a diameter of 

 j^'", in Mellita only -^-q'". They are extremely numerous, much 



