DIUESTIVK SYSTEM OF FISHES. 231 



by a sphincter ; in the Lepidosiren it is further defended by a soft 

 valvidar fold like an epiglottis.* 



The alimentary canal is usually short, simple, but capacious in 

 fishes ; in a few instances, e. g. Branchiostoma ( Jig. 46. ph, as), 

 Myzinoids (xxi. Neurologie, tab. iii. Jig. 6.), Exocetus, Lepidosiren 

 (xxxiii. j^>/. 25.), it extends in almost a straight line from the pharynx 

 to the anus : but it is generally disposed in folds and sometimes in 

 numerous convolutions. It is primarily divided into a gastric and 

 an intestinal portion by the constriction called 'pylorus.' The 

 gastric portion is subdivided into ' oesophagus ' and ' stomach,' the 

 boundary line being more commonly indicated by a change of struc- 

 ture of the lining membrane than by a cardiac constriction ; the in- 

 testinal portion is subdivided into a ' small ' and a ' large intestine ; ' 

 the latter usually answering to the 'intestinum rectum,' and the 

 boundary, when Avell defined, being a constriction and an internal 

 valvular fold ; but very rarely marked by an external cascum. 



The alimentary canal is situated wholly or in part in the abdominal 

 cavity, to the walls of which it is usually suspended by mesogastric 

 and mesenteric duplicatures of the peritoneal lining membrane of the 

 abdomen. When not wholly so situated, the extra-abdominal part 

 is not contained in a thoracic division of the cavity, but extends 

 beyond the jjeritoneal region into the muscular mass of the tail ; a 

 portion of the intestines, for example, lies between the right myo- 

 commata and the hosmal spines in the Sole. The peritoneal serous 

 membrane, which defines the abdominal cavity, extends anteriorly to 

 the pericardium, from which it is separated by a double aponeurotic 

 septum {fig. 61. o) : it is continued along the back over the ventral 

 sui'face of the kidneys and the air-bladder, when this exists, a little 

 way beyond the anus, and is reflected upon the alimentary canal, 

 (ib. d. i), the liver {I I), the spleen (fi), the pancreas (k), or its 

 cajcal rudiments, the ovaria or testes, and the urinary bladder, if 

 this be present. In many fishes the peritoneum does not form a shut 

 sac, but communicates with the external surface, by one (Bnnwhios- 

 toma, fig. 46. od, Lepidosiren, xxxiii. pi. 25. fig. 1. a), or two 

 (Lamprey, fig. 74. /, Eel, Salmon, Sturgeon, Planirostra, Chim;vra, 

 and Plagiostomes, figs. 73. and 75. I), orifices, situated, except in the 

 Lancelet, in or near the cloaca. The peritoneal orifices give exit to 

 the generative products (milt or roe) in the Lancelet, Myxinoids, 

 Lampreys, Murainidaj, and Salinonidaj, but not in the Lepidosiren and 

 Plagiostomes. In the INIyxinoids, the Amraocetes, the Sturgeon, the 

 Chimiiera^ and the Plagiostomes, the peritoneum communicates also 

 with the pericardium, j 



* XXXIII. p. 312. i\g.j.d. f I. XIX. J)]. iS. 



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