FISHES. 



319 



although the skeleton shows a bent back-bone (fig. 120, 

 C,D). 



FIG. 120. Different forms of tails of fshes. .4, diphycercal; B, hetero- 

 cercal; C, D, homocercal. (From Zittel.) 



There are two nasal sacs, although frequently four 

 nostrils are present, the four arising by a closure of the 

 two primitive openings in the middle. Inside the sac is 



