PISCES. 213 



are, nevertteless, furnished with an organ which doubtless 

 corresponds to (or is homologous with) the lungs of the higher 

 Vertebrata. This is known as the " air " or " swim bladder," 

 and is a sac filled with gas and situated between the alimen- 

 tary canal and the kidneys. In most cases, the sac contains 

 • only a single cavity, but, in many instances, it is variously 

 divided by partitions. In most fresh-water fishes, the gases in 

 the swim-bladder are mainly composed of nitrogen, but, in the 

 sea fishes, it is chiefly filled with oxj'gen. The sac of the 

 swim-bladder is often closed, but, in other cases, it opens into 

 the gullet by means of a duct which corresponds to the wind- 

 pipe. In the great majority of fishes, the functions of the 

 air-bladder are mainly hydrostatic, that is to say, it serves to 

 maintain the necessary agreement between the specific gravity 

 of the fish and that of the surrounding water. In the singu- 

 lar Mud-fish {Lepidosiren), the air-bladder is composed of two 

 distinct sacs, divided into a number of cellular compartments, 

 and opening into the gullet by a tube. In this fish it acts as 

 a respiratory organ, and is, therefore, not only in structure, but 

 also in function, the representativ^e of the lungs of the other 

 Vertebrates. 



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

 ganization, the brain being of comparatively small size, and 

 consisting mainly of a collection of ganglia. As regards the or- 

 gans of the senses, two peculiarities deserve notice. In the first 

 place, though fishes possess the essential parts of the organ 

 of hearing, they possess no external ears, and in no case is 

 there any direct communication between the ear and the outer 

 world. In the second place, the organs of smell consist of a 

 double cavity lined by a mucous membrane folded into numer- 

 ous plaits, into which water is admitted, usually by two dis- 

 tinct apertures or nostrils. Behind, however, the nasal sacs 

 are closed, and they do not communicate by any aperture with 

 the throat, as they do in all the higher Vertebrates. The only 

 exceptions to this rule are the Hag-fishes and their allies 

 {Myxinoids) and the Mud-fish (Lepidosiren). 



As regards their reproductive system, most fishes are truly 

 oviparous, and the ovaries are familiarly known as the " roe." 

 Some fishes are ovo-viviparous, retaining their eggs within the 

 body till the young are hatched. The male organs of repro- 

 duction are commonly spoken of as the " milt " or " soft roe." 



