39& MANUAL OF ZOOLOGY. 



doubtless the homologue of the lungs of the air-breathing 

 Vertebrates. This the " air " or " swim-bladder " is a sac 

 containing gas, situated beneath the alimentary 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 maintain the necessary 

 accordance between the specific gravity of the fish and that of 

 the surrounding water. In the singular Mud-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 sub- 

 divided by septa. In the Mud-fish the air-bladder is composed 

 of two sacs, completely separate from one another, and divided 

 into a number of cellular compartments. The duct leading in 

 many fishes from the air-bladder (ductus pneumaticus) opens 

 into the oesophagus, and is the homologue of the wind-pipe 

 (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 principally at the surface. 



The nervous system of fishes is of an inferior type of organi- 

 sation, the brain being of small size, and consisting mainly of 

 ganglia devoted to the special senses. As regards the special 

 senses, there is one peculiarity which deserves special notice, 

 and this is the conformation of the nasal sacs. The cavity of 

 the nose is usually double, and is lined by an olfactory mem- 

 brane, folded so as to form numerous plicae. Anteriorly, 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 statement are 

 to be found in the Myxinoids and in the Lepidosiren. The 

 essential portion of the organ of hearing (labyrinth) is present 

 in almost all fishes, but in none is there any direct communi- 

 cation between the ear and the external medium. In some 

 cases, however, there is a communication between the ear and 

 the swim-bladder, thus foreshadowing the Eustachian tube in 

 man. 



As regards their reproductive system, fishes are, for the most 

 part, truly oviparous, the ovaries being familiarly known as the 

 " roe." The testes of the male are commonly called the " soft 

 roe" or " milt." The products of the reproductive organs are 



