(ESOPHAGUS, STOMACH, AND INTESTINE. 



229' 



in which it has a spiral form, and is therefore called the spiral 

 valve. In the last-named Fishes, it begins to undergo degeneration 

 (Fig. 187, sp.v), and is no longer met with in other Vertebrates. 1 



The pyloric caeca (appendices pyloricse), which are charac- 

 teristic of the intestine of many Fishes, belong to the same physio- 

 logical category as the above. They are met with in Ganoids 

 and numerous Teleosteans, and consist of longer or shorter finger- 

 shaped processes of the small intestine, which are situated posteriorly 



FIG. 186. INTESTINAL TRACT OF A SHARK. 



H, heart ; PC, pericardium cut through ; Sv, sinus venosus ; L, L, the two lobes of the 

 liver, separated from one another so that the stomach (M ), with its pyloric tube 

 (PR), and the region of the pylorus (P) are visible ; MD, small intestine, in'which 

 the spiral valve lies; ED, large intestine; Gsp, rectal gland; AT, cloacal 

 pockets ; Pa, Pa, abdominal pores ; Pn, pancreas. 



to the pylorus in the region of the bile-duct (Figs. 187, c, and 188, 

 Ap). Their number varies from 1 (Polypterus and Ammodytes) 

 to 191 (Scomber scombrus). The pyloric caeca and the spiral valve 

 seem to be developed in inverse proportion to one another, for, 



1 Amongst the Teleostei a spiral valve is present in Cheirocentr us, and pro- 

 bably also in B u t i r i n u s. An intestinal valve exists also amongst the S a 1 m o n i d a?. 



