64 



CLASS PISCES. 



buccal cavity, and the lateral walls of the pharynx are pierced by 

 the gill-slits. Following the pharyngeal cavity, there is a usually 

 short funnel-shaped oesophagus, and a large stomach, which is 

 frequently drawn out into a caecum of considerable size (Fig. 37). 

 Caecal appendages (pyloric appendages) are not unfrequently 

 met with at the entrance of the lower mid-gut (small intestine) 

 which is marked off by a valve ; they probably serve the purpose 

 of increasing the extent of the secreting surface of the alimentary 

 canal. The intestine is usually several times coiled, and its 

 internal surface is remarkable for the longitudinal folds of the 

 mucous membrane ; villi such as are found in the higher Verte- 

 brates are only rarely present ; but in the Selachians, Ganoids, 

 and Dipnoi there is a peculiar spirally-coiled longitudinal fold — 



Fig. 37. — Alimentary canal and generative organs of Clupea harengus (after Brandt). A 

 anus : Ap pyloric appendages ; Br gills ; £> intestine ; Dp pneumatic duct ; Gp genital 

 pore ; Oe oesophagus ; S spleen ; T testis ; V stomach ; Vd vas deferens ; Fn swimming 

 bladder. 



the so-called spiral valve — which contributes essentially to the 

 enlargement of the absorbent surfaces. A rectum is not always 

 clearly marked off, and when present is always short, and in the 

 Selachians it is furnished with a caecal appendage. The anus is 

 usually situated far back, and is always ventral, and in front of 

 the urinary and generative openings, when the latter do not 

 lead unto the rectum (cloaca). In fishes with jugular pehic 

 fins, and in some Teleosteans without pelvic fins, it is situated 

 very far forward, and may even be on the throat. 



Salivary glands are absent in fishes, but there is a large liver 

 which is rich in fat and is usually provided with a gall-bladder ; 

 there is also usually a pancreas, which is by no means replaced 

 in Teleosteans by the pyloric appendages as was formerly be- 

 lieved. 



