64 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



around the posterior part of the oesophagus. The salivary ducts are 

 long, and enter the same fissure of the buccal body at which the cesoph- 

 agus issues. The 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. 



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 trans- 

 verse folds, corresponding to the contractions which produce the saccu- 

 lated appearance of the organ ; and in several species of different genera 

 it presents a few longitudinal rugee, as in Limax flavus, Liguus fasci- 

 atus, Mesodon exoleta, etc. In its whole extent it is formed of a colum- 

 nar epithelium and a nucleolated-nucleated basement membrane. The 

 columnar cells of the epithelium are long and pyramidal, the upper 

 part or base being broad, and the attached extremity very narrow. 

 They are filled with a very fine, indistinct, granular matter, inter- 

 mingled with coarser, highly reflective granules. Each contains an 

 oval, granular nucleus, with a minute nucleolus. 



The muscular investment of the intestinal canal is strongest upon 

 the stomach and rectum. In Veronicella, Strophia incana, etc. it is 

 strong and shining upon the stomach. It consists of two layers, an 

 internal transverse and an external longitudinal. They are both com- 

 posed of white, shining, strap-shaped bands, with the exti'emities 

 pointed and closely adapted to each other. None of the transverse 

 bands surround the stomach, all being much too short. They are in- 

 distinctly granular in structure, and each contains one or two elongated 

 nuclei. 



The lobules of the salivary gland are composed of the dilated com- 

 mencements of the ducts, lined with soft, glanular cells, which are oval 

 in form, and contain a roundj granular nucleus with a minute nucleolus. 

 The basement membrane of the salivary ducts is amorphous. The epi- 

 thelial cells lining the trunks bear considerable resemblance to those 

 found in their follicular commencement. Outside of the basement 

 membrane, twine narrow muscular fibres in various directions. They 

 are nucleated, and where the nuclei exist are wider than at the inter- 

 vening parts. 



The lobuli of the liver are composed of the rounded commencement 

 of the biliary ducts, and are lined with polygonal cells, which become 



