FISHES. 377 



they do not require extended members for their support. Their 

 general form is elongated, but thicker in the middle than at the 

 extremities; and the tail, the principal instrument in their pro- 

 gressive motion, is always long, and terminated by a vertical fin, 

 capable of expansion and contraction like a fan. The organs 

 of locomotion assume in this class the form of fins, of which 

 those named thoracic or pectoral, from their situation on the 

 body, have been considered as analogous to the fore-feet of 

 quadrupeds ; and those placed further backwards, called ab- 

 dominal or ventral fins, have been conceived to represent the 

 hind feet of the first class of vertebrated animals. The vertical 

 fins on the back are termed dorsal fins, and those on the under 

 surface of the body anal fins; the fin by which the tail is terminat- 

 ed being termed the caudal fin. The membranes of these fins 

 are supported by rays or bands more or less numerous; and 

 those of the pectoral and ventral fins, according to the suppos- 

 ed analogy between the organs of fishes and quadrupeds, have 

 been supposed to represent the toes of the feet. Other rays, 

 placed at the extremity of the spinous processes, support the ver- 

 tical fins of the back, those under the tail, and at its extremity. 

 These radii or rays are of two kinds ; those of a solid, bony, and 

 pointed form, sometimes flexible and elastic, are called spinous 

 rays ; and those composed of a number of small joints, generally 

 divided into branches at their extremity, are called soft or ar- 

 ticulated rays. 



In the number of the members as many varieties are observed 

 among the fishes as among the reptiles ; for though generally 

 speaking the fishes have four, yet in some families there are 

 but two, and others want them altogether. 



The greater part of fishes make their progressive motion in the 

 water by means of their expanded tail, which, striking the water 

 alternately from right to left, impels themforward; and they change 

 their direction by striking more rapidly, or with greater force on 

 one side than another. The fins which are in pairs appear to 

 be intended, besides aiding in progression, to maintain their 

 equilibrium in the water, and to regulate the direction of their 

 course. Some families, however, as the Rays, swim chiefly by 

 means of these fins ; but the particular form of the body must 

 naturally influence their medium of locomotion. 



