FISHES 243 



Here, as in all breathing organs, we find a large extent of surface, 

 thin membranes, and rich blood supply, all adaptations for osmotic 

 exchange, together with protective devices in the form of operculum 

 and gill rakers, and provision for a free circulation of water. 



Trunk. Extending along both sides of the body backward from 

 the operculum is a row of pitted scales with sense organs beneath 

 them, known as the lateral line, which probably aids the ears in 

 feeling vibrations, and functions as a pressure organ to estimate 

 the depth at which they swim. The fins are the most characteristic 

 and noticeable appendages of the trunk and consist of a double 

 membrane, supported by cartilaginous or spiny rays, and operated 

 by powerful muscles. Their shape and number vary with the kind 

 of fish, but there are always two pairs, the pectoral (anterior) and 

 pelvic (posterior) fins, which are homologous with the arms and 

 legs of other vertebrates. The other fins are all on the median 

 (middle) line of the trunk, there being sometimes two dorsal fins; 

 always a large tail (caudal) fin, and an anal fin just back of the 

 vent. In general the fins are beautifully fitted for locomotion 

 in the water, but they are differently used in this process, the caudal 

 fin being the chief propelling and steering organ. The paired 

 fins aid in locomotion and in balancing, and also support the body 

 when resting on the bottom. The other median fins aid in steering 

 and are often provided with sharp spines for defense as well. 



The bulk of the fish's body consists of powerful muscles. The 

 flexible backbone is made up of very numerous vertebrae, which, 

 together, permit the fins to be utilized to the fullest extent and 

 provide a system of aquatic locomotion, second to none in the 

 world, aided as it is by the pointed, scale-covered, slippery 

 body. 



Internal Structure. Digestive System. The food of most fishes 

 consists of other aquatic animals, though a few are vegetarians. 

 It is grasped by the mouth, but the teeth serve only for prehension 

 and not for chewing. On this account the gullet is large and short, 

 the stomach provided with powerful digestive fluids and usually 

 with finger-like outgrowths (caeca) to increase the digestive surface. 

 As in most carnivorous animals, the intestine is rather short, 



