126 



THE ELASMOBRANCH FISHES 



observed that the folds are far apart anteriorly and very much closer together 

 posteriorly. The valve is formed as an ingrowth into the intestine and extends 

 from the duodenum throughout the large intestine to the region where the 

 opening of the rectal gland enters the intestine posteriorly. This fold is con- 

 siderably broader than the diameter of the intestine and is thrown into a series 

 of cones having their apices pointing anteriorly. The surface of the valve, if 

 seen under the microscope, shows numerous finger-like villi which serve for the 

 absorption of digested food. 



Colon and Rectum 



The part of the large intestine immediately following the valve is known as the 

 colon (co., fig. 120). It is a muscular segment and superficially appears as 

 slightly bulbous. Its lining, together with that of the part succeeding it, the 

 rectum (re), is thrown into longitudinal folds. In the most posterior part, 

 however, the walls of the rectum are smoother. The line of demarcation be- 

 tween the colon and the rectum is formed by a 

 lumen from the rectal or digitiform gland. 



EECTAL GLAND 



The rectal or digitiform gland (rc.g., fig. 120 and 

 fig. 124) in Heptanchus is a finger-like structure 

 which is composed of multitudes of gland cells 

 and which empties by a central lumen into the in- 

 testine. It is so arranged, however, that the lumen 

 does not enter immediately at the point at which 

 it reaches the intestine, but passes sharply for- 

 ward and downward emptying on a level with the 

 terminus of the spiral valve. 



CLOACA 



Fig. 124. Sagittal section 

 through rectal gland, Hep- 

 tanchus. (From Howes.) 



CO., colon; fd., fold of spi- 

 ral intestine; lu., lumen of The rectum empties into an enlarged room, the 

 gland ; re, rectum. , i • i • t -, ^.^ , i 



cloaca, which is lined with a smooth mucous mem- 

 brane (see fig. 252, facing p. 290). Into the anterior part of the cloaca empty 

 the products from the digestive, the urinary, and the genital systems, and in its 

 posterior part are two finger-like processes, the cloacal papillae (p.). 



ABDOMINAL PORES (POEI ABDOMINALES) 



As a usual thing the cloacal papillae in Heptanchus are imperforate. Occa- 

 sionally, however, as on the left side in figure 252, they are perforate, forming 

 the so-called pori abdominales. These pores connect the abdominal cavity or 

 coelom with the exterior. 



