26 
MOLLUSCA. 
Heteropoda 
Have tlieir branchiae on the back, 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 branchise 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 ia Vermetus, &c., the foot is recurved in such a manner 
that the operculum is before. 
(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 ii). 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 assierned to it bv the author. — 
Eng. Ed. 
