GASTEROPODA OF THE UNITED STATES. 21 



anterior lobes of the liver are comparatively very small, 

 while the posterior lobe is correspondingly large ; and to 

 the whole there is but a single duct. 



Chapter v. — observations on the tissues of the 



digestive apparatus. 



The mucous membrane of the alimentary canal is 

 usually smooth throughout. In the stomach it frequently 

 presents a number of transverse folds, corresponding to 

 the contractions which produce the sacculated appearance 

 of the organ ; and in several species of different genera 

 it presents a few longitudinal rug^, as in Limax varie- 

 gata, Bulimus fasciatus, Helix exoleta, etc. In its 

 whole extent it is formed of a columnar epithelium and 

 a nucleolated-nucleated basement membrane. The col- 

 umnar cells of the epithelium are long and pyramidal, 

 the upper part or base being broad, and the attached ex- 

 tremity very narrow. They are filled with a very fine, 

 indistinct, granular matter, intermingled with coarser, 

 highly reflective granules. Each contains an oval, gran- 

 ular nucleus, with a minute nucleolus. 



The muscular investment of the intestinal canal is 

 strongest upon the stomach and rectum. In Vaginulus, 

 Pupa incana, etc. it is strong and shining upon the sto- 

 mach. It consists of two layers, an internal transverse 

 and an external longitudinal. They are both composed 

 of white, shining, strap-shaped bands, with the extremities 

 pointed and closely adapted to each other. None of 



