360 ZOOLOGY 



6. No true (allantoic, see Fig. 108, al.) urinary bladder. 



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

 line") on each side of the body. 



8. Nasal pit not (usually) communicating posteriorly with 

 the mouth. 



9. Fertilization and development usually occurring outside 

 the body. 



379. Fonn. — 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. Those which enter crevices under rocks or 

 elsewhere, as eels and catfish, are cylindrical. Some types are 

 extremely freakish in form, as the spheripal globe fish, angler- 

 fish which is chiefly head, the strange hammer-headed shark, 

 the pointed gar-pike and sword-fish, and the seahorse. 



The head end, while not so specialized as in the higher forms, 

 is much more cephalized than in the Protovertebrata. 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. 



380. Appendages. — Fishes possess two classes of append- 

 ages — paired, or lateral, and median. These are expansions 

 of the skin supported by rays of bone or cartilage. The paired 

 fins are four in number and are considered to be homologous 

 with the pectoral and pelvic appendages of the higher verte- 

 brates. They differ much as to their position, especially the 

 posterior pair, as may be seen by a comparison of the figur.es in 

 this chapter. In its typical condition the appendage consists 

 of girdle and the fin proper. In the lung-fishes there is a central 

 axis (Fig. 177) through the fin, instead of the usual radiate 

 arrangement (Fig. 17-8, f.r.) of the fin-rays. The legs of higher 



