12 Prof. J. ^Miiller on the Structure of the Ecbiiioderms. 



petaloid ambulacrum of the anterior pair only 28 ; the second 40 ; 

 the thirdj 44 pairs of pores : in each ambulacrum of the posterior 

 pair the first has only 14, the second 21, the third 25 pairs of 

 pores on each side. 



The ClypeasteridcB have, while the Cidaridce have not, a per- 

 manent equatorial periphery. If an old and a young Echinus 

 of the same species be compared, those plates which in the 

 young specimen lay in the equator of the periphery, have in the 

 older ones moved towards its ventral side; and the equator is 

 occupied by a circlet of plates which in youth lay near the apex. 



If the shell of the Sea-urchin terminate at some distance from 

 the mouth and the oral feet are seated upon the oral integu- 

 ment, both the inter-ambulacral and the ambulacral plates ter- 

 minate in pairs, as in Echinus. If, however, the shell is con- 

 tinuous up to the mouth and the oral feet are situated upon the 

 shell itself, then the ambulacral plates terminate in pairs, while 

 the inter-ambulacrals are single, as in the Spatangidoi and Chj- 

 peasterida. At the oral aperture in the Chjpeasterida the corona 

 becomes^ it is true, considerably simplified, but not so much as 

 is commonly supposed; in the end we have either a circlet of 

 fifteen pieces, five of which are inter-ambulacral {Clypeaster, 

 Mellita), or ten ambulacral pieces [Arachnoides) . It is requisite 

 to examine young specimens, though the sutures may always be 

 recognized upon the inner surface. In Clypeaster, the first 

 circlet around the mouth is composed of fifteen pieces, the second, 

 on the other hand, only of ten, — the ambulacral plates becoming 

 mutually applied, which is a generic character for all the species : 

 in a few species the third circle also is completely formed by the 

 ambulacral plates ; further out, two alternating series of inter- 

 ambulacral plates lie between them. 



At the oral aperture of the shell of the ClypeasteridcB with 

 branched clefts, there is a small process at the commencement of 

 each ambulacrum, noticed, in fact, by Agassiz in his beautiful 

 monograph upon the Scutellida, and regarded by him as a tube 

 with one or many apertures for the reception of the gills. Upon 

 this process, however, only two small tentaculiform processes 

 with rounded ends like the feet are attached. These little ten- 

 tacles are fixed to two shallow depressions of the process, from 

 each of which a fine pore, similar to an ambulacral pore, leads 

 obliquely into the interior of the shell. In Arachnoides placoita 

 the process is absent, but the apertures exist at the edge of the 

 shell, separated by the width of the groove. To these apertures 

 the two anterior branches of the ambulacral vessel run, whence 

 it is perfectly demonstrated that they are ambulacral and not 

 branchial, and that they correspond with the oral feet of the 

 regular Sea-urchins. The gills of Echinus are replaced in all 



