116 A COURSE ON ZOOLOGY. 



of the body, and represent the limbs ; the others are in 

 the median line, and are more or less directly connected 

 with the spinal column. In the first of these varieties 

 of fins are found, greatly modified it is true, the bones 

 that form the limbs of mammals. The median fins are 

 supported by little rods or rays of a bony nature. 



The fins that correspond to the upper limbs are called 

 pectoral fins. They are absent in but a small number 

 of fishes ; they are usually in pairs, nearly always in- 

 dependent of each other, and are situated near the 

 gills. 



The fins corresponding to the lower limbs are named 

 ventral fins ; they are absent in some fishes, among which 

 is the eel, and in those in which they exist they are 

 placed either under the throat, as in the codfish and 

 whiting, below the pectoral fins, as in the perch and 

 mullet, or under the abdomen. The latter is the case 

 in the carp, in which these fins have ten or eleven 

 rays. 



Of the median fins the most noticeable is the dorsal, 

 which is missing in some fishes, and presents great modifi- 

 cation in form and number in the different species. The 

 carp has one large dorsal fin, of which the first ray is 

 strong and notched like a saw, and succeeded by eighteen 

 or twenty others that support the fin. 



At the extremity of the vertebral column a terminal 

 forked fin constitutes the tail of the fish. Lastly, near 

 the anus, and always in the median line, is the anal fin, 

 the anterior ray of which is saw-toothed. 



Fish swim in water as birds fly in the air, that is, 

 they exert, by the aid of their organs of locomotion, a 

 pressure upon the fluid that surrounds them sufficient to 

 enable them to offer firm resistance. The posterior por- 



