32 The Dissection of the Fish 
sosteus) the valve is reduced to a rudiment of three or four con- 
volutions near the end of the intestine. In the sharks and 
skates the intestine opens into a cloaca, which contains also 
the urogenital openings. In all fishes the latter lie behind the 
orifice of the intestine. In the bony fishes and the ganoids 
there is no cloaca. 
Length of the Intestine.—In all fishes, as in the higher ver- 
tebrates, the length of the alimentary canal is coordinated with 
the food of the fish. In those which feed upon plants the intes- 
Fic. 20.—Stone-roller, Campostoma anomalum (Rafinesque). Family Cyprinide. 
Showing nuptial tubercles and intestines coiled about the air-bladder. 
tine is very long and much convoluted, while in those which 
feed on other fishes it is always relatively short. In the 
stone-roller, a fresh-water minnow (Campostoma) found in the 
Mississippi Valley, the excessively long intestines filled with 
vegetable matter are wound spool-fashion about the large air- 
bladder. In all other fishes the air-bladder lies on the dorsal 
side of the intestinal canal. 
Fic. 21.—Skeleton of the Cow-fish, Lactophrys tricornis (Linnzeus). 
