Prof. J. Miiller on the Structure of the Echinoderms. 115 



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 Asterida, beneath the trunks of 

 the ambulacral vessels and nerves. On the other hand, the ex- 

 ternal table of the ambulacral plates lies over the trunks of the 

 nerves and vessels, like the membranous covering of the ambu- 

 lacra of the Asterida. Herein we have sufficient evidence that 

 in fact the structure of the ambulacra in the Echviidce and Aste- 

 rida is widely different, and Cidaris and Echinantkv.s may be con- 

 sidered to furnish the key to the proper understanding of these 

 deviations. 



The Ophiunda depart a step further than the Asterida from 

 the Sea-urchins. The ambulacral plates have still retained their 

 vertebral form in the Ophiuridae, and the ambulacral canal runs 

 in a groove over them ; above the ambulacral vessel, however, 

 lies the flat nenous cord of the arm, a ad above that are the pecu- 

 liar plates, the ventral discs of the arms ; but under the vertebral 

 portions or analogues of the ambulacral plates there are no am- 

 pullae, the latter structures being totally absent in the Ophiurida. 

 The lateral branches of the ambulacral vessel pierce the ventral 

 portion of the vertebral segment horizontally as far as the suckers, 

 which are arranged along a groove of this part of the skeleton. 

 Pores leading to internal diverticula, comparable to the ambu- 

 lacral pores of the Asterida and Sea-urchins, are non-existent. 

 The nervous trunk of the arm gives a branch to every sucker, for 

 which an appropriate groove is excavated upon the ventral sur- 

 face of the vertebral segment. 



In comparing the Sea-urchins with the Asterida, particular in- 

 terest attaches to the five plates of the apex of the former, which, 

 from their position between the genital plates, have been called 

 intergenital plates — a term long in use, for which Agassiz has 

 lately substituted the name of ocellar plates, which I think 

 almost too theoretical to be safely used. Each of these plates is 

 situated at the end of an ambulacrum without being itself an 

 ambulacral plate; it is pierced, and in the aperture the ocular 

 bulb discovered by Forbes is situated. This body, the fact of 

 whose existence has been confirmed by Agassiz and Valentin, 

 and which I also have seen (in Cidaris), is the analogue of the 

 coloured ocular spot discovered by Ehrenberg at the extremity of 

 the arms of the Asterida. In both cases the nerAOus cord of the 

 radius enters the bulb, passing in the Sea-urchins from within out- 

 wards, through the aperture in the plate. Agassiz justly lays very 

 great weight on this analogy, and ascribes to the Asterida also 

 an ocellar plate at the end of the ambulacrum, between which 

 and the ambulacrum the new ambulacral plates are formed, as in 

 the Sea-urchins. Here also the new inter-ambulacral plates 



8* 



