BRANCH CHORD AT A; CLASS PISCES: THE HSHES ?73 



On the outside of the head are numerous bones called 

 membrane bones, because they are made up of ossified 

 membrane. The most important of these is the operclc 

 or gill-cover. Within are the tongue with the fivejjill- 

 arches attached to it below and to the floor of the skull 

 a5ove~ the last arch being usually modified to form the 

 pharyngeal jaw. 



The^stomach may be a blind sac with entrance and exit 

 close together, or it may have the form of a tube or 

 siphon. At its end are often found the large glandular 

 tubes called pyloric caeca which secrete a digestive fluid ; 

 and to its right side is attached the red spleen. Theliver 

 is large, having usually, but not always, a gall-bladder; 

 it "pouTs its secretion into the upper intestine. In fishes 

 which feed on plants the intestine is long, but it is short 

 in those which eat flesh, because flesh is digested in the 

 stomach, not in the intestines. The kidney is usually a 

 long slender forked gland showing little variation. The 

 egg-glands differ greatly in different sorts of fishes, the size 

 and number of eggs varying equally. The air-bladder is 

 a lung which has lost both lung structure and respiratory 

 function, being simply a sac filled with gas secreted from 

 the blood, and lying in the upper part of the abdominal 

 cavity. It is subject to many variations. In the gar 

 pike, bow-fin and the lung-fishes of the tropics, the air- 

 bladder is a true lung used for breathing and connected 

 by a sort of glottis with the oesophagus. In others it is 

 rudimentary or even wholly wanting, while in still others 

 its function as an air-sac is especially pronounced, and in 

 many it is joined through the modified bones of the neck 

 to the organ of hearing. 



The blood of the fish is purified by circulation through 

 its gills. These are a series of slender filaments attached 

 to bony arches. Among them the blood flows in and out, 

 coming in contact with the water which the fish takes in 



