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 

 branchiae. The stomach is full of cells, the bottom of each 

 pierced with a biliary duct. A singular organ, termed the 

 crystalline 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 posteriorly by a tubular anus. 



The branchiae consist of two ribbands on each side, ex- 



