10 



S. LOVEN, ON POUKTALESIA, A GENUS OF EOIIINOinEA. 



II. THE PERISOMATIC SYSTEM. 



The perisomatic system in the Cystoidea and in tlie Echinoidea. The anoraalous disposition of its ele- 

 ments in tiie Ponrtalesiadas approaching to annnlose diflferentiation; the heteronoiny of 1 and 4 maiiitained; 

 the periproct, its position in Pouitalesia similar to its position in the Cassidulida^. The fasciola. The spines. 



The perisome is the general envelope, and the interradia — the »area3» of Linnaean 

 terminology — are the portions of it that are exposed to view between the arnbulacra 

 and outside the calycinal system. It alone makes up the whole skeletal säck of some 

 Cystoidea. In Callocystites it is easily seen to be continuous under the ambulacra, 

 which are attached solely by their first adoral plates, but otherwise free. Its sutures 

 are clearly observable running under them, and its sui^face is marked with impressions 



Callocystites Jewetti Hall. 



Restored: showing the marks left ou the perisome by the slightly 

 raised ambulacra. 



Ambalacrum broken away to show 

 the marks. 



caused by their backs, when at rest and reclining against it. The independent natui'e 

 of the two systems cannot be more clearly indicated. Supposed then, as seems at pre- 

 sent quite lawful, that those movable members, which in the Cj^stoidea bear the oral 

 grooves issuing from the corners of the mouth, are homologous to the ambulacra — 

 the fettered limbs — of the Echinoidea, the question arises whether it may not be pos- 

 sible some day by skilful manipulation to demonstrate, in some species or other, the 

 uninterrupted continuation, under the ambulacra, of the interradia, as a very thin mem- 

 brane. But this is only one of the many questions to be taken up by a thorough in- 

 vestigation of the histology of Echinodermata. Another is that concerning the relation 

 of the perisomatic system to the calj^cinal. In the Cystoidea, — in which every trace of 

 a calyx is wanting, at least in the adult — the basal part of the skeleton is formed by 



