MOLLUSCA. 501 



towards the mouth, is usually tongue-shaped, capable of 

 considerable elongation, with a furrow on its posterior sur- 

 face. This organ, where a byssus is present, is considered 

 as employed in spinning and fixing the threads. When 

 there is no byssus, it either acts as a sucker, enabling the 

 animal to crawl along the surface of bodies, or as a paw, 

 to dig holes in the sand or mud. None of the species can 

 float in the water ; they either crawl or leap, the last kind 

 of motion being effected by suddenly opening and shutting 

 the valves. In securing a residence, some of the species 

 bore into different substances by means of a rotatory motion 

 of the shell It was at one time supposed that the dwelling 

 was formed by a secretion affecting the solution of the sur- 

 rounding substance. But the very different substances pe- 

 netrated 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, in order to understand the relative si- 

 tuation of the parts, here to state, that, upon laying the ani- 

 mal upon its back, and opening the cloak, the abdomen ap- 

 pears to occupy the middle longitudinally, and the branchia 

 to be arranged on each side. The mouth is situate at the 

 anterior extremity, and consists of a simple aperture enter- 

 ing into the gullet, or rather stomach. It is surrounded 

 by four flattened ^joveable tentacula, two of which, in some 



