Prof. J. Miiller on the Structure of the Echinoderms. 117 



terids, which is superficial and uncovered by any hard structure. 

 In the vermiform Asterid-larva which I have described, the 

 csecal extremities of the ambulacral canals project from the ex- 

 tremities of the arms, in which respect this larva agrees much 

 more closely with the Ophiuridce than with the Aster idee. We 

 are not yet acquainted with its earliest condition, and it is very 

 desirable to ascertain whether its form resembles a Pluteus or a 

 Bipinnaria. There is no evidence to enable us to say whether 

 the terminal segment of its vermiform body should be referred 

 to the anus or madreporic plate of the body of a starfish. If it 

 were an Ophiurid, this segment must be considered as the 

 remains of the dome of the larva. 



The terminal articulation of the arm of an Ophiura is neither 

 ambulacral nor purely anti-ambulacral, but as it were an osseous 

 node, from which the ambulacral plates, the ventral and dorsal or 

 anti-ambulacral discs, and the inter-ambulacral lateral discs take 

 their origin. The terminal plate of the arm of the Asterid may 

 be considered to be such a node. The ocellar plate of the Sea- 

 urchin is the terminal plate of a radius whose antambulacral 

 side is absent. In analysing the radii of Disaster into a trivium 

 and bivium, the genital plates, of which there are four, remain 

 at the apex of the trivium ; the ocellar plates however follow 

 their radii. 



Oral Skeleton of Echinidse, Holothuriadse and Ophiuridse. 



According to H. Mayer's elaborate analysis the oral skeleton 

 of the regular Echinidce consists of five pairs of alveoli for the 

 five enamelled teeth, of ten epiphyses for the inter-connexion of the 

 former, and of five other radial pieces upon which the epiphyses 

 articulate. These are the pieces which Des Moulins calls rotulce, 

 Valentin f aloes. An additional suspensory apparatus of the oral 

 skeleton is constituted by the five 'compasses' of Valentin, 

 which have been shown by Mayer to consist each of two por- 

 tions ; they are present only in the regular Sea-urchins, and are 

 totally absent in the Clypeasterida, 



The two epiphyses of each pair of alveoli are, in the regular 

 Sea-urchins, provided with processes, which in Echinus, &c., 

 become united into an arch at the base of the alveoli ; in Cidaris 

 and Echinocidaris the processes are present, but no longer united 

 into arches ; in Diadema the processes of the epiphyses have en- 

 tirely vanished. In the Echini therefore, the ten epiphyses form 

 with the five rotulse a continuous circle ; in Cidaris, Echinocidaris 

 and Diadema an interrupted one. In the * Anatomische Studien 

 Uber die Echinodcrmen ' I have compared this circle with the 



