VERTEBRATA: FISHES. 



475 



them are, nevertheless, furnished with an organ which has 

 been generally believed to be the homologue of the lungs of 

 the air-breathing Vertebrates. This the "air" or "swim 

 bladder" is a sac containing gas, situated beneath the ali- 

 mentary tube, and often communicating with the gullet by a 

 duct. In the great majority of fishes the functions of the air- , 

 bladder are certainly hydrostatic that is to say, it serves to I 

 maintain the necessary accordance between the specific gravity 

 of the fish and that of the surrounding water. In the singular 

 Mud-fishes, (as also in a few Bony Fishes), however, it acts as 

 a respiratory organ, and is therefore not only the homologue, 

 but also the analogue, of the lungs of the higher Vertebrates. 

 In most fishes the air-bladder is an elongated sac with a single 

 cavity, but in many cases it is variously subdivided by septa, or 

 it may give off more or less complicated caeca (fig. 258). In 

 the Mud-fishes the air-bladder is composed 

 of two sacs, completely separate from one 

 another, and divided into a number of 

 cellular compartments. The duct (ductus 

 pneumaticus) leading in many fishes from 

 the air-bladder, and opening into the oeso- 

 phagus, is the homologue of the windpipe 

 (trachea). The air contained in the swim- 

 bladder is composed mainly of nitrogen in 

 most fresh-water fishes, but in the sea- 

 fishes it is mainly made up of oxygen. 

 The fishes which live habitually at the 

 bottom of the sea, such as the Flat-fishes, 

 possess no swim-bladder, and it is much 

 reduced in size in those which live prin- 

 cipally at the surface. 



The nervous system of fishes is of an 

 inferior type of organisation, the brain be- 

 ing of small size, and consisting mainly 

 of ganglia devoted to the special senses. Fig. 258. Swim-bladder 

 As regards the special senses, there is So^^SSSS 

 one peculiarity which deserves particular 

 notice, and this is the conformation' of the nasal sacs. The 

 cavity of the nose is usually double, and is lined by an olfac- 

 tory membrane, folded so as to form numerous plicae. An- 

 teriorly, the water is admitted into the nasal sacs by a single 

 or double nostril, usually by two apertures; but posteriorly 

 the nasal sacs are closed, and do not communicate with the 

 pharynx by any aperture. The only exceptions to this state- 

 ment are to be found in the Myxinoids and in the Mud-fishes. 



