10 BULLETIN OF THE BUREAU OF FISHERIES. 
OBSERVATIONS AND EXPERIMENTS. 
HISTOLOGY OF THE ALIMENTARY CANAL OF THE SMOOTH DOGFISH AND THE 
SAND SHARK. 
The intestine of both JMustelus canis and Carcharias littoralis is bent twice upon 
itself; the first of these bends is between the stomachic sac and the pyloric tube, the 
second between the pyloric tube and the middle intestine. From an anatomical 
standpoint these two bends divide the alimentary tube into three portions, of which 
the first two constitute the anterior intestine, the third the middle and terminal 
intestine. As arule the middle intestine, or what we might call the duodenum, is 
short. In J/ustelus canis, indeed, there is almost no duodenum or valve-free portion 
between the pyloric tube and the spiral valve. On the other hand, in Carcharias 
littoralis, Carcharhinus obscurus, Dasyatis centrura, Lamna cornubica, and Tetronarce 
occidentalis, the middle intestine, or duodenum, is well marked off from the spiral 
valve and pyloric tube. From a histological standpoint the entire intestine may be 
divided into buccal, esophageal, stomachic, pyloric, duodenal, valvular, rectal, and 
cloacal mucous membrane. 
Upon the digestive tract of European selachians, as Yung shows in his paper, 
“*Recherches sur la digestion des Poissons” (1899), considerable histological work 
has been done. Yung himself made a thorough study of the alimentary canal of 
Seyllium canicula. The histology of Mustelus canis and Carcharias littoralis is 
practically the same and agrees in most respects with that of the European form 
Scyllium canicula. Histological study of the digestive tract of the American 
species shows the following facts: 4 
BUCCAL MUCOUS MEMBRANE. 
The mucous membrane of the buccal cavity is smooth, often covered with fine 
papillze and moistened with mucus. Sections of the mucous membrane of the buccal 
cavity showed epithelium and connective tissue, but noglands. The epithelium is of 
the stratified pavement type. The epithelial cells next to the connective tissue are 
cylindrical, finely granular, and possess oval nuclei. Above this layer of cylindrical 
cells are several layers of large mucous cells, which are oval and contain a substance 
that stains with the ordinary mucus stains. The nucleus is very small, elongated, 
and pressed against the cell wall. Finally, the superficial epithelium consists of one 
or two layers of flat or oval cells which form a fine membrane. 
MUCOUS LINING OF THE ESOPHAGUS. 
Numerous papille and longitudinal folds occur in the inner lining of the esoph- 
agus. The folds are fine at their beginning, but thicken toward the cardiac end of 
the stomach. They vary in number and frequently anastomose. Transverse folds 
form a boundary more or less marked between the esophagus and the stomach. The 
