35 6 ZOOLOGY. 



5. Vertebral column either cartilaginous or bony; the verte- 

 brae biconcave. 



6. There is no true (allantoic, see Fig. 205, a/.) urinary 

 bladder. 



7. A longitudinal line of sense organs (" organs of the 

 lateral line ") on each side of the body. 



8. Nasal pit does not (usually) communicate posteriorly 

 with the mouth. 



9. Fertilization and development usually take place out- 

 side the body. 



372. Form. Fishes usually have a body somewhat flattened 

 from side to side, though it may be quite cylindrical or flat- 

 tened dorso-ventrally, and gradually tapering toward either 

 end. This is readily seen to be a form well suited to motion 

 through water. The head end, while not so specialized as in 

 the higher forms, is much more cephalized than in the Proto- 

 vertebrata. The mouth with its modifications of movable 

 jaws, teeth, etc., the respiratory arches, the sense organs of 

 sight, hearing, taste, and possibly smell, the brain and brain 

 case all enter into this cephalization. 



There is no neck, i. e. } the head is not movable with refer- 

 ence to the body. The length of the body varies very greatly 

 as does the number of metameres embraced. The body may 

 be distinguished as pre-caudal and caudal. 



373. Appendages. Fishes possess two classes of append- 

 ages paired, or lateral, and median. The paired fins are 

 four in number and are considered to be homologous with 

 the pectoral and pelvic appendages of the higher vertebrates. 

 They differ much as to their position, especially the posterior 

 pair, as may be seen by a comparison of the figures in this 

 chapter. In its typical condition the appendage consists of 

 girdle and the fin proper. The skeletal supports may be either 

 cartilaginous or bony. In the lung-fishes there is a central 

 axis (Fig. 174) through the fin, instead of the usual radiate 

 arrangement (Fig. 175, f.r.) of the fin-rays. The legs of higher 



