352 MANUAL OF ZOOLOGY. 



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 nervous system of Fishes is of an inferior type of organ- 

 isation, 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. 



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 

 often set free into the peritoneal cavity, ultimately finding their 

 way to the external medium, either by means of an abdominal 

 pore (or pores), or by being taken up by the open mouths of 

 the " Fallopian tubes." In other cases the generative pro- 

 ducts are directly conveyed to the exterior by the proper ducts 

 of the reproductive organs. 



