14 ASTEROIDEA. 



and Jsferiada are differently, and, in fact, so differently disposed, as to give rise to the 

 main distinctive peculiarities of a sea-urchin and of a star-fish. 



Still greater are the differences between the ambulacra of the AderiadcB and EcJiinidcs 

 in the vertical direction. The nervous cord and the ambulacral canal of the Jsferiada 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 Echinida, however, they lie 

 beneath the ambulacral plates on the inner surface of the shell. The vertebral processes of 

 the ambulacral plates of the Asferiada are absent in most EcMnidce ; but in the Cidarida 

 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 intervertebral 

 passages, apertures for the branches given off by the ambulacral vessel to the ampulla3 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 extremity of the corona of 

 the sea-urchins with the vertebral processes of the Asferiada, which is remarked in the 

 " Anatomische Studien liber die Echinodermen" (' Archiv.,' 1850), is more apparent 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 alius (or the genus Echinantlms altogether) 

 possess that part of tlie ambulacral plates which is analogous to the vertebral 

 processes of the Asferiada;, in the internal table of their ambulacral plates. In this case 

 all the ambulacral plates take a part in its formation, and the right and left portions 

 are even united by a suture. This ambulacral floor lies, as in the Asferiada, beneath 

 the trunks of the ambulacral vessels and nerves. On the other hand, the external 

 table of the ambulacral plates lies over the trunks of the nerves and vessels, like the 

 membranous covering of the ambulacra of the Asferiada. Herein we have sufficient 

 evidence that, in fact, the structure of the ambulacra in the Echinida and Asferiada 

 is widely different, and Cidaris and Echinanflnis may be considered to furnish the 

 key to the proper understanding of these deviations.^ 



' I doubt the accuracy of this statement, for my dissections of Uraster ruhens, Lin., showed that the 

 course of the principal nerve of the ray was along the middle of the upper part of the ambulacra arches, 

 the position homologous to that which the nerve occupies in the Echinidce, namely, beneath the ambulacral 

 plates on the inner surface of the shell, as stated in the text by Miiller. 



2 Johannes MuUer, ' Ueber den Bau der Echinodermen,' 4to, plates, Berlin, 18.54. 



' Annals and Magazine of Natural History,' 2d series, vol. xiii, pp. 1 13-) 15. See a translation of parts 

 of the above work by Professor Huxley, from which the above extract is taken. 



