336 



COMPARATIVE ANATOMY 



- -a.V 



-st 



FIG. 245. ALIMENTARY VIS- 

 CERA AND SWIM-BLADDER 

 OF Lepidosteus (in situ). 

 (After Balfour and Parker.) 



a, anus; a. b. swim-bladder; 

 a.6 1 , its aperture into the 

 throat ; b.d 1 , aperture of 

 bile-duct into intestine ; 

 e, pyloric caeca; y.b, gall- 

 bladder ; hp.d, hepatic 

 duct ; lr, liver ; py, pyloric 

 valve ; s, spleen ; sp.v, 

 spiral valve ; st, stomach. 



1 In the Holocephali there 

 the rectum encloses numerous 



Pyloric cceca are met with in Ganoids 

 (except Amia) and most Teleosts, and 

 consist of longer or shorter finger- 

 shaped processes of the small intestine, 

 situated posteriorly to the pylorus in 

 the region of the bile-duct (Figs. 245, 

 246). Their number varies from 1 

 (Polypterus and Ammodytes) to 191 

 (Scomber scomber) : in some fishes they 

 are bound together by connective tissue 

 so as to form a compact mass. The 

 Shark Lsemargus possesses a pair of 

 caeca opening into the anterior part of 

 the intestine. 



In the narrow-bodied Gymnophiona 

 the intestine is only slightly coiled, 

 while in Anura it becomes considerably 

 folded on itself: its form in Sala- 

 manders is about mid-way between 

 these two extremes. 



In the Cyclostomi, Holocephali, 

 Ganoidei, and most Teleostei, there is 

 a separate anus; in all other Fishes 

 as well as in the Amphibia the large 

 intestine opens into a cloaca, common 

 to it and to the urinogenital ducts. 

 The large intestine (rectum) is com- 

 paratively short and takes a straight 

 course ; in Amphibians, as well as to 

 some extent in certain Ganoids and 

 Teleosts, it is plainly marked off from 

 the small intestine by its size, and be- 

 tween the two there is often a circular 

 valve. In some cases the rectum is 

 considerably swollen and may even 

 exceed the stomach in capacity (Fig. 

 249). An outgrowth of the ventral 

 wall of the cloaca in Amphibia gives 

 rise to the urinary bladder and repre- 

 sents the allantois (q.v.) of higher forms. 

 In Plagiostomes a finger-shaped 

 rectal gland (processus digitiformis) l 

 opens into the anterior part of the 

 rectum, and this perhaps corresponds 

 to the blind gut or caecum of higher 

 forms. Traces of a caecum are seen in 

 certain Teleosts (e.g. Box). In the 



is no processus digitiformis, but the thick wall of 

 gland-tubules. 



