118 Prof. J. Miiller on the Structwe of the Echinoderms. 



oral ring of the Holothuriadce. The rotulse or radii of the Lantern 

 resemble those pieces of the calcareous ring in the Holothuriadce , 

 over which the five ambulacral canals pass outwards ; in the Sea- 

 urchins they have the same relation to the five ambulacral canals. 

 The Holothuriad(B have neither alveoli nor teeth. 



In the Clypeasterida the oral apparatus has exactly the same 

 composition as in the regular Sea-urchins, with a somewhat dif- 

 ferent form of the epiphyses and rotulse. That their oral skeleton 

 possesses only the five pair of jaws, the remaining parts being 

 wholly absent, as Agassiz states with regard to those genera de- 

 scribed in his monograph upon the Scutellida, is true of no genus 

 of this family. On the other hand, in all the genera of Clypeas- 

 tendce^ this apparatus consists of twenty-five calcareous pieces, 

 viz. ten semi-alveoli ^ their ten articular epiphyses, and five rotula. 

 The rotulse of the Clypeasters were observed by Des Moulins, 

 who, however, failed to find them in the other genera. Their 

 form diff'ers from that of the regular Sea-urchins, inasmuch as 

 they are deep and disc-shaped ; the ambulacral canal passes as 

 usual beneath them and above the inter-alveolar muscles_, to the 

 circular canal. The articular epiphyses of the alveoli are con- 

 nected with them by a joint, and they hold the alveoli sufficiently 

 apart to prevent the ambulacral canal, which passes under the 

 rotula, from being compressed by the action of the inter-alveolar 

 muscles. In most genera of the Clypeasteridce the epiphyses of 

 the alveoli have almost the same form as the rotulse, and are 

 united by sutures with the alveoli. Des Moulins has not recog- 

 nised the epiphyses, but Don Antonio Parra observed both the 

 epiphyses and the rotulse in Clypeaster rosaceus, stating that 

 there are three small pieces between the alveoli : — " En la union 

 de dos de estas piezas, por la parte superior, dexan un hueco en el 

 que estan calocadas maravillosamente tres piececitas, de figura 

 de la pepita de un melon verde, estas se designan por la fig. 8.'^ 

 (Descripcion de difierentes piezas de historia natural, Havana, 

 1787, p. 141.) So that the structure of the oral skeleton in the 

 Clypeasteridce was well understood in the last century, and long 

 before that of the regular Sea-urchins. 



The genera of the Clypeasteridce all possess the same pieces, 

 and are distinguished merely by the form of the alveoli and the 

 position of their articulating surfaces, which in Clypeaster and 

 Arachnoides are nearer the oral cavity, in the others are at the 

 external angles of the jaws. In Arachnoides placenta, however, 

 the rotulse are remarkable for their excessive and unusual pre- 

 dominance in size over the epiphyses and the elevation of their 

 bases above the alveoli, while the epiphyses are small and have 

 the ordinary form. In Lobophora both the rotulse and the epi- 



