26 MOLLUSCA, 



HETEROPODA 



Have their branchiae on the hack, where they form a transverse 

 range of small panaches, protected, as well as part of the viscera, in 

 some species, by a symmetrical shell. They are particularly distin- 

 guished, however, by the foot, which is compressed into a thin 

 vertical fin, on whose margin is frequently observed a small cup 

 (yentouse), the only vestige of the horizontal foot of the rest of the 

 class. In the 



PECTINIBRANCHIATA 



The sexes are separated ; the respiratory organs almost always con- 

 sist of branchiae, composed of lamellae, united in the form of combs, 

 and are concealed in a dorsal cavity, widely open above the head. 



Nearly all of them had a turbinated shell, a mouth sometimes 

 entire, sometimes fissured, and at other times furnished with a siphon, 

 but most generally susceptible of being more or less perfectly closed 

 by an operculum attached to the foot of the animal behind*. The 



TuBULIBRANCHIATA(a) 



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 Pectinibranchiata ; but the sexes 

 are united, so that fecundation takes place without a mutual copula- 

 tion, as in the Acephala. Their shell is very open, and in several 

 forms a non-turbinated shield; the operculum is always wanting. 

 The 



CYCLOBRANCHIATA, 



Hermaphrodites, like the Scutibranchiata, have a shell composed of 

 one or several pieces, but never turbinated nor with an operculum ; 



* N.B. Sometimes, as in Vermetus, &c., the foot is recurved in such a manner 

 that the operculum is before. 



Q^ (a) In the original this order does not occur, but we find further on, that 

 when the author comes to take each of these orders into detailed consideration, as it 

 will be seen he does in the following pages, the necessity occurred to him of sepa- 

 rating from the Pectinibranchia an additional order, to which he gave the name of 

 of Tubulibranchia. We have therefore deemed it necessary to insert this order 

 with its characters precisely in the order and relation assigned to it by the author. 

 ENO. ED. 



