Prof. J. Mullci* on the Slruciurc of the Echinodci'ms. 13 



the Sea-urchins with i)etaloid ambulacra, as well Spatangidce as 

 Cli/peasteridce, by the ambulacral gills of the petaloid ambulacra ; 

 however, the Spatangidce have well-developed oral tentacles. 



In the regular Sea-urchins of the genera Echinus, Echino- 

 metra, Salmacis, and others, all the feet, including those of 

 the oral disc, have a similar structure, being suckers with a 

 sucking disc ; but this is not the case with every regular Sea- 

 urchin; they are not all, to use Duvernoy's term, homoiopodous. 

 Delle Chiaje, indeed, has already stated that the dorsal feet of 

 Echinus neapolitanus {Echinocidaris cequituberculata) are pecti- 

 nated, though his figure does not give a correct representation 

 of them. The fact is, that all the Echinocidarides present this 

 peculiarity. The lower feet have a sucking disc below, which is 

 a circular calcareous ring. On the dorsal portion of the ambu- 

 lacrum, the sucking disc and the calcareous ring suddenly vanish 

 altogether, the feet becoming at the same time laterally flattened, 

 pointed at their ends, and lobed on their flat sides. This arrange- 

 ment, which would appear to be repeated in Asteropyga and 

 Diadema (from dried specimens), evidently affords a transition 

 to the gdl-like dorsal feet of the Spatangidce. In Colobocentrus 

 atratus, likewise, the feet undergo a metamorphosis from the 

 ventral to the dorsal sides ; the suckers gradually disapjjear, and 

 the feet take, upon the dorsal surface, a flat pointed form without 

 lobes, very different from that in Echinometra. These feet con- 

 tain two canals separated by a partition, but uniting with one 

 another at the extremity, while at the base each is connected 

 with one of the double pores. All previously mentioned sea- 

 urchins have the ordinary cutaneous gills at the anterior edge 

 of the corona, like Echinus {Diadema and Asteropyga, also, if 

 we may judge by the insections of the shell). They are absent 

 in Cidaris, the ventral feet here being cylindrical with terminal 

 suckers, while on the dorsum they are conical and not lobed. 



The oral feet of Cidaris form complete series upon the move- 

 able buccal plates, which here, in a manner, repeat the corona, 

 and may be divided into ambulacral buccal plates with double 

 pores and inter-ambulacral buccal plates, the former of which 

 remain double up to the mouth, while the latter become simple 

 at the furthermost extremity. 



The SpatangidcE present far more numerous diff'erences in the 

 characters of the ambulacral feet. Four general forms may be 

 distinguished : — 1. Simple locomotive feet, truncated or slightly 

 rounded, without any special sucking disc. 2. Locomotive feet 

 terminated by a sucker; this is either a large round disc notched 

 at its edges and supported by radiating reticulated calcareous 

 plates, or its edge is divided, star-like, into digitations, the 



