THE ELASMOBRANCH FISHES 



141 



Colon and Rectum 



The segments of the intestine succeeding the valvular intestine, the colon and 

 the rectum, usually differ in their lining from the valvular intestine in the ab- 

 sence of villi. Separating these two areas and emptying into the dorsal side of 

 the digestive tract is the rectal gland. 



The rectal or digitiform gland varies in size from a tiny structure one-half 

 an inch long, as in some of the rays, to one three or four inches long in some of 

 the larger sharks. It is a compound, tubular 

 structure, the secreting cells of which are 

 surrounded by a strong, fibremuscular layer. 

 These cells empty their secretion through 

 tubules into a median lumen which, as we 

 have seen in Heptanchus, enters the intes- 

 tine between the colon and the rectum. In 

 some of the rays the lumen is large, that part 

 at which it enters the intestine being espe- 

 cially expanded (Raja, fig. 141b) . In most of 

 the sharks, however, the gland is constricted 

 at its base, and its lumen is small. As a gen- 

 eral thing the lumen is prolonged forward 

 so that the entrance to the intestine is ante- 

 rior to the position of the gland (Zygaena, 

 fig. 141a). Although the rectal gland has 

 been studied in a great number of forms, its 

 function has not yet been made out. 



CLOACA 



The cloaca is the common receptacle into 

 which the digestive tract and the ui'inary 

 and genital systems empty. It is generally an 

 enlarged room, the walls of which are more 

 or less loosely folded. In some types, trans- 

 verse crescentic folds from the dorsal wall of 

 the cloaca separate it into an anterior and a 

 posterior division on each side. In the ante- 

 rior division two types of structure may be 

 found. One of these is a pair of finger-like 



papillae (see fig. 252, 2^., facing p. 290) ; the other is a pair of cloacal pits. The 

 papillae, when present, usually contain prolongations of the lining of the body 

 cavity. The cloacal pits are often located lateral and posterior to the papillae. 

 In some forms both the pits and the papillae are present. In connection with 

 the pits or with the papillae may occur apertures, the abdominal pores, which 

 put the abdominal cavity into connection with the outside. 



Fig. 140. Valvular intestine of Eaja. 

 (From Paul Mayer.) 



