CONCHIFERA. 71 



CLASS II. CONCHIFERA. 

 (Mollusca Acephala, Cuv.) 



Animal soft, inarticulated, destitute of head or eyes, and al- 

 ways Jixed in a bivalve shell ; branchice external ; circula- 

 tion simple ; heart unilocular. 



THE Animals of this class have no apparent head; and their 

 mouth, concealed under the mantle, or at the junction of its two 

 lobes, and destitute of jaws or hard parts, appears but as the ori- 

 fice of a short oasophagus. The mantle or cloak which envelopes 

 thebodyis large, in two lobes, and incloses the trunklike the cover 

 of a book. In some families, however, this mantle is united be- 

 fore, and then forms a tubular covering, open at both ends. The 

 mantle besides often forms two tubes or syphons, of which one 

 conducts the water to the branchiae, and the other serves as a 

 canal for dejections. This mantle is always furnished with a 

 shell of two valves, united by a hinge or ligament ; and strong 

 transverse muscles, attached to the shell, enable the animal to 

 open or shut it at pleasure. 



The nervous system in this class is imperfectly developed, 

 sensation very obtuse, and the brain, if such it may be termed, 

 is a ganglion over the mouth, formed by the junction of two 

 nervous chords. Their chief sense seems to be that of touch. 

 In some families this sense appears to reside in tentacular fi- 

 laments which border the lobes of the mantle, or certain places 

 of these lobes. These tentacular threads, which appear very sen- 

 sible, or at least irritable, are in general numerous, short, very fine, 

 and move sometimes with extreme quickness. 



The heart in the Conchifera is placed towards the back. It 

 is small, but provided with venous and arterial vessels. The 

 liver is large, embracing the stomach and a great portion of the 

 alimentary canal. The branchiae are external, and appear more 

 particularly so in those in which the mantle opens before. These 

 branchiae are opposite, formed of large vascular leaflets, gene- 

 rally crescent-shaped, placed on each side under the cloak, cov- 

 ering the belly of the animal, upon the sides of which they are 



