FISHES 271 



When a fish is alive, however, it can compress or expand its air- 

 bladder by the action of its muscles, and therefore rise and sink at will 

 within certain limits. If it sinks too far, however, the pressure of the 

 water may be so great that the air-bladder cannot expand sufficiently to 

 cause it to rise again, and if it rises too far the pressure of the water 

 may be so slight that it cannot compress its air-bladder again. We do 

 not know how far the living fish may be able to accommodate itself by 

 increasing or decreasing the quantity of air in the air-bladder. We know 

 that many bottom fishes, like the plaice and sole, and all the Selachii, 

 are destitute of an air-bladder. 



4. The Skeleton is bony in the larger number of fishes, the bony 

 fishes, or Teleostei. In the other orders (which see) it remains carti- 

 laginous throughout life. We shall here confine ourselves to the former 

 of these orders. 



(a) The body of a fish being of nearly the same specific gravity as the 

 water, and accordingly supported by the latter, it follows that the bones 

 which compose the vertebral column and limbs are far weaker than those 

 of land animals or birds, who have to support the weight of their own body. 

 For the same reason the pectoral and pelvic arches also are only feebly 

 developed. The former of these, as a rule, is connected with the skull ; 

 the latter is composed of a single bone, which is never connected to the 

 vertebral column. These conditions, and the absence of a neck, also 

 explain the remarkable uniformity of the vertebral column, in which 

 only two segments, an abdominal and a caudal segment, are distinguish- 

 able. The abdominal segment is usually characterized by the presence 

 of a large number of ribs, which form a protecting framework for the 

 abdominal cavity and the viscera. Embedded among the large dorsal 

 and lateral muscular bands are also to be found generally a larger or 

 smaller number of bones, the intermuscular spinous processes of the ribs. 

 In correlation with the mode of progression of fishes, we find that the 

 vertebral column, and especially its caudal segment, is very flexible from 

 side to side. All the vertebras present conical cavities at both ends, 

 being biconcave or amphicoelous, and only in contact with each other at 

 the outer margins of these cavities. (For the fin rays and interspinous 

 bones refer back to Section 2, a.) 



(b) The limbs of fishes not being adapted for the seizing of their 

 prey, this task is necessarily performed (as in snakes) by the mouth. 

 The anterior, facial portions of the skull, i.e., the maxilla and the pre- 

 maxilla, are protrusible to a considerable extent, as may be easily 

 observed in such fish as the carp. (Compare, on the other hand, the 

 shark. Premaxillary bones occur also in the other classes of vertebrates ; 

 in mammals they carry the incisor teeth.) Teeth are present in all parts 



