FISHES. 



503 



to the lower extremity of the body, which is called the anal fin. 

 These fins are always nearly of the same structure, consisting gene- 

 rally of a fold of the skin, supported by slender, flexible, cartilaginous 

 or osseous rays, connected by a thin membrane. 



Fig. 340. Skeleton of the Common Perch. 



a, the inter-maxillary bone ; b, the maxillary bone ; d, the gills ; c, the under jaw ; /, the inter-oper- 

 culuni ; g g, the vertebral column ; h, the pectoral fin; i, the ventral fin ; k and I, the dorsal fins ; m, the 

 anal fin ; n, the caudal fin. 



The muscles which bind together the vertebral column are so 

 much developed in fishes as well as others of the superior animals, 

 that they constitute in them alone the principal part of the body. The 

 caudal, dorsal, and anal fins act as outlying oars ; the pectoral and 

 ventral fins assist in progression, at the same time that they help to 

 maintain the equilibrium of the animal and guide and direct its move- 

 ments, which are generally astonishing from their rapidity. 



F/'g. 341. Swimming bladder of the Carp. 



An organ, which belongs properly to fishes* (Fig. 341), and which is 

 usually considered as their chief aid in swimming, is a large bladder 

 situated within the body, between the dorsal spine and the abdomen. 

 This is usually called the swimming bladder. According to the 



* Some fish, as tho Chondropterygii, have no swimming bladder. ED. 



