114 Prof. J. Miiller on the Sti-mture of the Echinoderms. 



marks off the dorsal pore-area from the ventral surface is the 

 equivalent of the marginal plates. In these forms also the 

 number of the series of plates from the groove of the arm to the 

 pore-area varies very greatly ; in some there are only two series 

 of plates, the intermediate plates disappearing, as in Echinaster 

 and Scytaster, whilst in Ophidiaster there are many series of 

 plates between the groove of the arm and the pore-area, the 

 outermost of which, as adambulacral plates and marginal plates, 

 extend completely to the extremity of the arm, the others, as in- 

 termediate rows of plates, are more or less, and, indeed, gradually, 

 diminished. It is obvious that the inter-ambulacral plates of 

 the Sea-urchins and Asterida are differently, and, in fact, so dif- 

 ferently disposed, as to give rise to the main distinctive pecu- 

 liarities of a Sea-urchin and of a Starfish. 



Still greater are the differences between the ambulacra of the 

 AsteridcE and Echinida in the vertical direction. The nervous 

 cord and the ambulacral canal of the Asterida lie, covered by the 

 integument, over the mutually applied ambulacral plates, that is, 

 upon the outer side of the vertebral processes of these plates ; in 

 the Echinidce, however, they lie beneath the ambulacral plates on 

 the inner surface of the shell. The vertebral processes of the am- 

 bulacral plates of the Asteridce are absent in most Echinida, but 

 in the Cidaridce they have a perfectly analogous structure at the 

 anterior extremity of the ambulacra, where the ambulacral plates 

 on the inner side of the series of pores send off perpendicular 

 processes into the cavity of the shell, between which lie the 

 trunks of the ambulacral organs. The ampullae are external. 

 The clavate ends of a number of these processes unite to form a 

 continuous colonnade, while they leave between their bases inter- 

 vertebral passages, apertures for the branches given off by the 

 ambulacral vessel to the ampullae and the pores of the shell. 

 There is no union of the vertebral processes of the right and left 

 side. The analogy of the auricular processes at the anterior ex- 

 tremity of the corona of the Sea-urchins with the vertebral pro- 

 cesses of the Asteridce, which is remarked in the ' Anatomische 

 Studien iiber die Echinodermen ' (Archiv, 1850), is more appa- 

 rent than universally true. The auricular processes are, indeed, 

 in most Sea-urchins, processes of the ambulacral plates, and the 

 ambulacral organs pass between them -, but in Cidaris we meet 

 with an exception, the inter-ambulacral plates giving off the 

 auricular processes for the muscles of the jaws. 



Besides Cidaris, Clypeaster rosaceus and altiis (or the genus 

 Echinanthus altogether) possess that part of the ambulacral 

 plates which is analogous to the vertebral processes of the Aste- 

 rida in the internal table of their ambulacral plates. In this 



