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



supposes the water to have access to the interior of the shell 

 (Comptes Eendus de PAcad. des Sciences de Paris, t. xxv. 679). 

 Hence we see that Agassiz was the first to observe these fine 

 pores and their ampullae, but that he confounded the proper 

 locomotive feet which are supplied from the ambulacral vessels 

 with respiratory tentacles. Neither in this nor in other Sea- 

 urchins are there any tubules which penetrate the shell to open 

 into the interior of the abdominal cavity. The ampullae in 

 question, in fact, are connected with the ambulacral vessels by 

 their branches to the locomotive feet ; and similar ampullae exist 

 also external to the petaloid ambulacra and upon the ventral sur- 

 face, connected with the branches of the ambulacral vessels and 

 ^j^rresponding with each locomotive pore. 



Relations of this kind appear to be prevalent among the Cly- 

 peasteridce, and obtain, not only in Echinarachnius parma, for spe- 

 cimens of which I am indebted to Prof. Eschricht, but in Cly- 

 peaster and Arachnoides, as results from the examination of the 

 specimens of Clypeaster placunarius collected by Messrs. Ehren- 

 berg and Hemprich, and in like manner in Clypeaster Rangi- 

 arms, and in the specimens of Arachnoides placenta brought by 

 Dr. Th. Philippi from Mergui. 



We may first examine the internal area of the petaloid ambu- 

 lacra. The whole area from one marginal double series of large 

 pores to the opposite double series of the petal is covered with 

 small cylindrical locomotive feet, which have the same size and 

 structure as the feet of the peripheral dorsal pore-areae, and as 

 the feet of the ventral pore-areae. The investigation of the in- 

 ternal surface of the shell throws further light upon the matter. 

 The median ambulacral canal in Clypeaster Rangianus, CI. pla- 

 cunarius, Arachnoides placenta, Echinarachnius parma, supplies 

 the ambulacrum with as many parallel lateral branches as there 

 are double pores for the ambulacral gills ; each lateral branch in 

 these is connected with the internal pore and with the large am- 

 pulla of the gill-like foot. But before reaching the internal pore, 

 an additional series of small caecal ampullae, of exactly the same 

 form as those of the peripheral dorsal and the ventral, locomotive 

 feet, is attached to the lateral vessel. Each of these small am- 

 pullae, of /q'" long and ^^jj"' broad, corresponds with a very small 

 aperture through which a twig of the transverse lateral vessel 

 penetrates the shell and passes to the locomotive foot upon the 

 external surface of the ambulacrum. In Clypeaster Rarigianus 

 there are twenty pores and ampullae in one transverse row of 

 half the ambulacrum in its broadest part, in Clypeaster rosaceus 

 thirty, in Clypeaster placunarius ten, in Arachnoides placenta 

 twenty, in Echinarachnius parma fifteen. In Echinarachnius 

 parma, whose ambulacral plates are similar, these locomotive pores 



