328 MOLLUSCA. 



of combs, and are concealed in a dorsal cavity, widely open 

 above the head. 



Nearly all of them have a turbinated shell, a mouth some- 

 times entire, sometimes furnished with a siphon, and most ge- 

 nerally susceptible of being more or less perfectly closed by 

 an operculum attached to the foot of the animal behind(l). 

 The 



TUBULIBRANCHIATA 



Have a shell resembling a more or less irregularly pointed 

 tube, which attaches itself to various bodies. Their branchiae 

 consist of a single range along the left side of the roof of the 

 branchial cavity. The 



SCUTIBRANCHIATA 



Have branchiae similar to those of the Pectinibranchiata ; but 

 the sexes are united, so that fecundation takes place without 

 a mutual copulation, as in the Acephala. Their shell is very 

 open, and in several forms a non-turbinated shield ; the oper- 

 culum is always wanting. The 



Cyclobranchiata, 



Hermaphrodites like the Scutibranchiata, have a shell com- 

 posed of one or several pieces, but never turbinated nor with 

 an operculum ; their branchiae are attached under the margin 

 of their mantle, as in the Inferobranchiata. 



ORDER I. 

 PULM0NEA(2). 

 The Pulmonea are distinguished from the other Mollusca 



(1) N.B. Sometimes, as in Vermetus, &.C., the foot is recurved in such a man- 

 ner that the operculum is before. 



(2) M. de Blainville prefers the term Pui-monobkanchiata. 



