ORGANS OF NUTRITION. 123 



ably uniform size witliiu a very short time ; for instance, the 

 Stickleback, many species of Gohius and Clupea. 



The organs of nutrition, manducation, and deglutition, are 

 lodged in two large cavities — an anterior (the mouth or huccal 

 cavity), and a posterior (the abdominal cavity). In the 

 former the alimentary organs are associated with those ful- 

 filling the respiratory functions, the transmission of food to 

 the stomach and of water to the gills being performed by 

 similar acts of deglutition. The abdominal cavity commences 

 immediately beliind the head, so, however, that an extremely 

 short thoracic cavity for the heart is partitioned off in front. 

 Beside the alimentary organs it contains also those of the 

 urogenital system and the air-bladder. The abdominal cavity 

 is generally situated in the trunk only, but in numerous fishes 

 it extends into the tail, being continued for some distance 

 along each side of the haemal apophyses. 



In numerous fishes the abdominal cavity opens outwards 

 by one or two openings. A single porus dbdominalis in front 

 of the vent is found in Lepidosiren and some Sturgeons ; a 

 pau-ed one, one on each side of the vent, in Geratodus, some 

 species of Sturgeon, Zepidosteus, Polypterus, Amia, and all 

 Chondropterygians. As in these fishes semen and ova are 

 discharged by proper ducts, the abdominal openings may serve 

 ■ for the expulsion of semen, and those ova only which, having 

 lost their way to the abdominal aperture of the oviduct, would 

 be retained in the abdominal cavity. In those Teleosteans which 

 lack an oviduct a single porus genitalis opens behind the vent. 



The mouth of fishes shows extreme variation with regard 

 to form, extent, and position. Generally opening in front, it 

 may be turned upwards, or may lie at the lower side of the 

 snout, as in most Chondropterygians, Sturgeons, and some 

 Teleosteans. Vogt regards this position as a persistent foetal 

 condition. In most fishes the jaws are covered by the skin, 



