12 BULLETIN OF THE BUREAU OF FISHERIES. 
half an inch to 2 inches in length. The latter is longer and very wide; it is divided 
into two portions—the colon, containing the spiral valve, and the rectum, which is 
short. 
From the end of the pyloric tube to the cloaca the histology of the intestine is 
practically the same and consists essentially of cylindrical and goblet cells. No 
glands are present, but the villi project into the lumen of the intestine both in the 
duodenum and in the spiral valve. The epithelium which covers these villi is the 
same throughout and consists of cylindrical and goblet cells. Since the villiare more 
prominent in the spiral valve, it would be well to consider this part of the intestine 
in detail. 
, SPIRAL VALVE. 
A spiral valve is present in the colon of cyclostomes, selachians, ganoids, and 
dipnoans. Its histological structure in Jfustelus canis and Carcharias littoralis is 
like that of the duodenum. The villi stop abruptly at the point where the rectal 
gland opens into the intestines. The folds of the spiral valve are formed from the 
mucosa of the walls of the intestines. Through the middle of each fold passes the 
muscularis. mucosa. From the center connective tissue extends into the villi. A 
cross section of a fold shows: (1) Epithelium of upper surface, (2) connective tissue, 
(8) connective tissue and muscular tissue, (£) connective tissue, (5) epithelium of 
undersurface. 
RECTAL GLAND. 
The rectal gland, glandula or processus digitiformis, is a compound tubular 
gland varying from one-half inch in the skate to four inches in the mackerel shark. 
It opens into the rectum by a duct, which, beginning at the central canal of the 
gland, runs forward along the edge of the mesentery to enter the dorsal wall of the 
lower end of the spiral valve or the top of the rectum. The gland consists of three 
layers: (1) an outer fibro-muscular layer, (2) a middle glandular layer, and (3) a central 
region consisting of ducts and blood vessels arranged round a central lumen. 
The middle layer is composed of a number of branched tubules radially arranged 
and separated by capillaries which are usually gorged with blood. The high power 
shows mono-nucleated cubical cells not clearly defined from each other and of a 
glandular appearance. 
The central layer begins ata varying distance from the periphery by the sudden 
transition of the gland cells into the epithelium of ducts which open into the central 
lumen. In many cases the more superficial cells have undergone a mucoid change 
and a band of clear cells is visible lining the duct. The microscopical appearance of 
the gland is shown in plate 1. 
PHYSIOLOGY OF THE DIGESTIVE TRACT OF ELASMOBRANCHS. 
While studying the food of the dogfish, J/ustelus canis, at the laboratory of the 
Bureau of Fisheries, Woods Hole, Mass., during the summer of 1904, Irving A. 
Field found that 16 per cent of the dogfish contained lobsters, 34.17 per cent rock 
crabs, and 20.1 per cent spider crabs. The carapace of these organisms consists of 
salts and chitin, the latter highly resistant to reagents. As the carapace was found 
in varying degrees of decomposition, and, further, since the carapace of crabs and 
