l6 CONCHOLOGIA CESTRICA. 



characters, they possess a very complicated and excitable 

 Muscular System, many of the muscles being united to the 

 skin. The ventral surface is covered with a thick muscu- 

 lar layer forming a long disc, which is termed the foot. 

 The contraction of these fibres produce wrinkles, which 

 succeed each other from behind forward, thereby enabling 

 the animal to glide over solid surfaces, or on the water. 

 In those possessing a turbinated shell, a large muscle 

 arises from the columella, and after dividing, is spread 

 over the sides of the body to be inserted into the foot ; 

 an arrangement which enables them rapidly to retract 

 the body within the shell. Numerous other muscles 

 within the body serve different uses in their economy. 



The Nervous System, is composed of several gangli- 

 onic masses, connected by nerve filaments. The largest 

 one is placed on the oesophagus, and is the functional 

 representative of the brain. 



The Circulation, is carried on by a heart consisting 

 of an auricle, and ventricle, usually placed on the right 

 side of the body, at the base of the branchia, in the 

 aquatic ; or in the pulmonary cavity of the terrestrial 

 species. The blood is usually bluish-white; in the 

 genus Planorbis it is red. 



The Organs of Respiration, are either pulmonary, or 

 branchial, and are for the most part lodged in a cavity 

 on the right side of the body, just within the last whorl 

 of the shell, the entrance to which is valvular, and may 

 be seen opening and shutting at the will of the animal. 



The Vent, is usually found just anterior to the pul- 

 monary orifice. A third opening, near the base of the 

 right tentacle, leads to the generative organs. 



The Tentacles, (mostly four,) are either cylindrical, 

 tubular, and retractile by inversion; or triangular, solid, 

 conical, and contractile, with the head beneath the 

 mantle. 



