80 MANUAL OF NATURAL HISTORY. 



such as the Mackarel, have this organ deeply forked 

 and pointed ; while in the more slowly-moving it is 

 square or rounded. The projection backwards of 

 water through the gill-aperture is also believed to 

 have some effect in assisting their forward progress, 

 which is also probably affected by the movements of 

 their various fins. These last-named organs are 

 among the most distinguishing objects of the class, 

 and mostly consist of thin, delicate membranes, 

 spread upon and supported by filaments or rays of 

 more or less power and flexibility. These may be 

 modified or divided in various ways, but an ac- 

 quaintance with their names and situations is requi- 

 site, as by their differences families and genera are 

 often separated. When complete they are seven in 

 number; viz., one on the back, named the dorsal; 

 two attached to the breast, the pectoral; two ven- 

 tral, one on either side of the belly; one anal, situ- 

 ated along the under surface of the body; and one 

 caudal, forming the tail. Of these the pectoral and 

 ventral fins may be looked on as analogous to the 

 fore and hind extremities of higher animals. The 

 dorsal fin was formerly considered as influencing the 

 perpendicular position of the fish in water, but from 

 some experiments mentioned by Yarrell,* the idea 

 would seem to be erroneous. The bodies of the 

 greater number are covered with scales, which, how- 

 ever, vary much in their size, shape, and arrange- 

 ment. These are attached by their anterior edge 

 being left free posteriorly. Four principal forms are 



* 2nd ed., vol. i. p. 365. 



