es 
LAMELLIBRANCHIATE CONCHIFERA. 89 
siderable opening of the valves by that organ; and in some species the valves them- 
selves are gaping inferiorly for its exsertion. 
There is a cartilage between the two small spinous processes of the hinge in the 
Pholas candidus. In other species of Pholas, which have no rudiment of it, and allied 
genera, which have a particular character of articulation, I consider the motion of the 
valves as but a secondary cause in the perforation of the substances in which these ani- 
mals are found. 
_ Muscular System. 
The gland for the formation of the byssus (the existence of which is denied by Blain- 
ville) may be found readily at the base of the foot (in the Modiola, for instance,) with 
a duct opening at the bottom of its groove. It is bilobate, and of a dark granular 
appearance. I consider the foot of the Pecten, Spondylus, &c. to be an organ of pre- 
hension of the food, collected by the vibratile currents near the mouth. In these it has 
but one long slender muscle. The uses of the foot as an organ of locomotion are de- 
scribed by naturalists. One use of the part called the crystalline stilette, I consider to 
be the giving resilience to the foot. It projects above into the stomach, and is directed 
inferiorly to the pore by which water is admitted into the foot, as it is in many species. 
The upper extremity has a small membranous cartilage upon it, called the tricuspid 
body. In the Anomia the stilette supports a free portion of the mantle; in the other 
Monomyaria it is absent ; the membrane is always present. The whole appears to be 
the analogue of the curious tongue of other Mollusca. 
Some species have two sacs of the mantle, containing sometimes a gaseous fluid, 
which is probably formed by the secreting organs, with which they are connected. 
These animals can thus lighten themselves, and easily change their situation at the ebb 
and flow of the tide. 
Nervous System. 
In more than twenty genera examined, I find a great similarity in the Nervous 
System. Poli, Cuvier, and Blainville are rather incorrect in their description of the 
nervous system of the Lamellibranchiata. Mangelli describes that of the Anodonta with 
great accuracy. 
When a foot is present, there are three ganglia or pairs of ganglia ; when absent, but 
two. These ganglia are of an orange colour externally, and white within. Two ganglha 
are situated at the mouth, more or less removed from each other, but always connected 
by a supra-cesophageal nerve ; they are sometimes on a level with, or before the mouth, 
sometimes behind it. They give off on each side filaments to the anterior muscle, ten- 
tacles, lips, and anterior part of the mantle. Each ganglion likewise gives off a twig 
going to the posterior ganglia, which are situated between the branchie on the posterior 
muscle. These are united into one, when the branchie are themselves united medianly, 
as in Mactra, Mya, Solen, &c., but are at a distance from each other, when the branchie 
are remote ; but in this case, are always connected by a transverse nerve, as in Modiola, 
nN 2 
