186 MANUAL OF ANIMAL BIOLOGY 



ileum. There is no external differentiation to be noted between 

 these two portions. The duodenum curves anteriorly and forms 

 with the stomach a U-shaped structure. When seen from the ven- 

 tral side, the duodenum forms the left side of the U. The common 

 bile duct carrying the materials from liver and pancreas opens into 

 the duodenum a short distance below the pylorus. The ileum 

 begins at the upper left-hand corner of the U and then curves 

 abruptly posteriorly, forms a number of coils, and finally enlarges 

 to form the large intestine. (W. f. 112.) 



A microscopical study of a prepared section through the wall of 

 the small intestine reveals the fact that it is composed of several 

 distinct layers of tissue which may now be described, (a) The 

 small intestine is covered by a thin layer, the serosa. (6) Below 

 the serosa is a double muscular layer composed of a thinner layer 

 of longitudinal muscle fibers and a thicker layer of circular muscle 

 fibers, (c) Within the muscular layer is the surmucosa, which 

 consists of connective tissue and vascular elements, (d) Finally, 

 the tube is lined by the essential mucosa, which is of endodermal 

 origin and consists of a single layer of cells, some of which are 

 specialized for the secretion of the digestive enzymes and some for 

 the absorption of the digested food materials. The wall of the 

 stomach has the same general arrangement. (W. f. 34.) 



Although, as previously stated, the essential structural features 

 of the alimentary canal remain uniform throughout the Verte- 

 brates, there are certain noteworthy modifications, some examples 

 of which may be noted. These may be regarded as adaptations to 

 the type of food or to the eating habits of the organism in ques- 

 tion. In certain Fishes the intestine is provided with devices 

 which retard the passage of materials and at the same time in- 

 crease the absorptive surfaces, just as has been seen in the typh- 

 losole of the Earthworm. The so-called spiral valve of the 

 Shark is a good example. It consists of a membranous fold of 

 tissue, which hangs down into the intestinal cavity. As the name 

 indicates, the attachment to the wall of the intestine is of a spiral 

 nature. The result is that the food materials in process of diges- 

 tion are forced around and around the interior of the intestine as 

 they are slowly moved posteriorly. In other Fish, as, for example, 

 in the Perch, the same result is obtained by the presence of several 

 blind sacs (pyloric caeca) which open from the small intestine 

 near the pyloric end of the stomach. (W. f. 106.) 



