194 MOLLUSCA. 
shell. It was at one time supposed that the dwelling was 
formed by a secretion affecting the’solution of the surround- 
ing substance. But the very different substances penetrat- 
ed by the same species, as limestone, slate-clay, and wood, 
forbid us to entertain such a supposition. 
The nervous system is here but little developed. The 
superior and inferior ganglia, surrounding the gullet, give 
rise to all the nervous filaments which proceed through the 
body. 
The digestive organs are scarcely less simple. The food 
is soft and swallowed entire, and either brought to the mouth 
by accident, or by eddies produced in the water by the 
opening and shutting of the shells, aided in some cases by 
the syphons. 
It may be proper here to state, in order to understand 
the relative situation of the parts, that, upon laying the ani- 
mal upon its back, and opening the cloak, the abdomen 
appears to occupy the middle longitudinally, and the bran- 
chia to be arranged on each side. The mouth is situated 
at the anterior extremity, and consists of a simple aperture 
entering into the gullet, or rather stomach. It is surround- 
ed by four flattened moveable tentacula, two of which in 
some are in part united with the cloak, while in others they 
are free to the base. In their structure they resemble the 
branchiz. The stomach is full of cells, the bottom of each 
pierced with a biliary duct. A singular organ, termed the 
erystalline process, cylindrical, cartilaginous, and transpar- 
ent, is found in some species projecting into the cavity of 
the stomach. The liver is large, surrounds the stomach, 
and pours out its contents by numerous openings. The in- 
testine terminates pesteriorly by a tubular anus. 
The branchiz consist of two ribbands on each side, ex- 
