PISCES 197 



the olfactory nerve, the nerve of smell, is distributed. This 

 cavity does not communicate with the mouth, as in the 

 Dipnoi and all the higher vertebrates. There is no external 

 ear. The scales, overlapping each other like the shingles of 

 a house, are calcified outgrowths of the skin to protect 

 the body. The unpaired dorsal fins vary from one to three. 

 A pair of pectoral fins is present just back of the gill open- 



FIG. 227. Dissected fish, a, air bladder ; b, urinary bladder ; 6r, brain ; c, spinal 

 cord ; d and da, dorsal fins ; dv, anal fin ; e, body of the vertebrae ; g, gills re- 

 ceiving the blood from the ventral aorta leading out of the heart h ; i, intestine ; 

 k, kidney ; I, liver ; m, spines of the vertebrae ; n, auricle of the heart ; ov, 

 ovary ; ovd, oviduct ; st, stomach ; sp, spleen ; M, ureter. Drawing by W. H. 



ing, and behind and below these is usually a pair of ventral 

 fins. The anal fin present in some forms is single, and 

 behind the opening of the alimentary canal. By removing 

 the entire ventral abdominal wall, most of the organs may 

 be seen. 



Teeth are usually present not only on the jaws, but in 

 some species on the vomer at the roof of the mouth and 

 on the gill arches. Vertical openings between the four 



