ORDER PECTINIBRANCHIATA. 53 
and several that are simply conical. ‘Their gills, composed of 
numerous lamelle or strips, laid parallel with each other, like 
the teeth of a comb, are attached on one, two, or three lines, 
according to the genus, to the ceiling of the pulmonary 
cavity, which occupies the last whorl of the shell, and which 
has a large opening between the edge of the mantle and the 
body. 
In two genera only, Cyclostoma and Helicina, do we find, 
instead of gills, a vascular net-work covering the ceiling of a 
cavity that is otherwise similar; they are the only ones that 
respire the natural air, all the others respire water. 
All the pectinibranchiata have two tentacula and two eyes, 
sometimes placed on particular pedicles, and a mouth, resem- 
bling a more or less elongated proboscis. The sexes are sepa- 
rate. The penis of the male, attached to the right side of the 
neck, cannot usually be retracted within the body, but is 
reflected into the cavity of the gills; itis sometimes very stout. 
The paludina is the only one which can retract it through an 
orifice perforated in its right tentaculum. The rectum and 
oviduct of the female also run along the right side of this 
cavity, and between them and the gills is a peculiar organ, 
composed of cells, from which exudes an extremely viscid 
fluid, serving for the formation of a common envelope which 
contains the eggs, and is deposited with them. ‘The figure of 
this envelope is often very complex and singular. 
Their tongue is armed with little hooks, and by slow and 
repeated rubbing acts upon the hardest bodies. 
The greatest difference in these animals consists in the pre- 
sence or absence of the little canal formed by a prolongation 
of the edge of the pulmonary cavity, on the left side, and 
which passes through a similar canal or emargination in the 
shell, to enable the animal to breathe without leaving its 
shelter. There is also this distinction between the genera, 
that some of them have no operculum. The species differ 
