10 



S. LOVEN, ON POURTALESIA, A GENUS OF EOIIINOIDEA. 



II. THE PERISOMATIC SYSTEM. 



The perisomatic system in the Cystoidea and in the Echinoidoa. The anomalous disposition of its ele- 

 ments in the Pourtalesiadne approachins: to annulose differentiation; the heteronomy of 1 and 4 maintained; 

 the poriproct, its position in Pourtaiesia similar to its position in the Cassiilnlidit;. The fusciola. The spines. 



The perisoioc is the general envelope, and the interradia — the »area3» of Linna^an 

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

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

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

 which are attached solely by their first adoral plates, but otherwise free. Its suture^s 

 are clearly observable running under them, and its surface is marked with impressions 



/ A 



Callocjstltes .Tewetti IIai.l. 



Restored: showing the marks left on the (lerisoine by the slightly 

 raised ambulacra. 



Ambiilacniin broken away to show 

 the marks. 



caused by their backs, when at rest and reclining against it. The independent nature 

 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 Cystoidea 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 cojitinuation, 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 liistologj' of Echinoderraata. Another is tliat concerning the relation 

 of the perisomatic system to the calycinal. 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 



