272 TEXT-BOOK OF ZOOLOGY 



of the mouth ; but as a constant stream of the water used for respiratory 

 purposes is maintained through the oral chamber, the food cannot be 

 subjected to mastication in the latter (discuss this more fully). Hence 

 the teeth can only be employed for holding the prey or biting or 

 tearing off morsels. (Why are not the teeth of various forms, as in 

 mammals ?) 



(c) The gill-covers (opercula), which consist of several bony pieces, and 

 the branchial arches, will be treated more fully in Section 5. We shall 

 here only note the following important points : The portions of the 

 branchial arches which come in contact with the skull are commonly 



beset with larger or smaller teeth, and from 

 the fact of their lying in the upper wall of the 

 pharynx are known as the superior pharyngeal 

 bones. Similarly, the fifth pair of branchial 

 arches which do not bear gills carry teeth on 

 their inner surfaces. The bones of this arch 

 perforate the throat from below, and are hence 

 termed the inferior pharyngeal bones. The 



i,,„ ,: iwyv^h. Pharyngeal bones, with their equipment^ of 



Bones of the Cam-, teeth, are used for holding the prey while it is 



being swallowed ; it is only in the carp family 

 (see carp) that they function as true organs of mastication. 



5. Respiration and Circulation. — If water be allowed to stand in a 

 glass for a considerable time, small vesicles of air will be observed to 

 congregate upon the sides of the glass, showing that the water really 

 contains air. If a fish be brought into water which has been allowed to 

 stand in this manner, or from which all air has been expelled by boiling, 

 it will die in a short time ; it expires from want of fresh air — is suffo- 

 cated, in fact. Mammals, birds, or reptiles are not capable of breathing 

 air thus dissolved in water, like a fish, for the latter possesses in its 

 gills or branchiae organs specially adapted for " aquatic respiration." 

 Let us now examine the structure of the gills and the process of branchial 

 respiration in one of our most familiar fresh-water fish — e.g., the perch 

 or carp. 



(a) On opening the mouth of one of these fish, it will be seen that the 

 walls of its posterior portion are perforated on each side by four cleft- 

 like apertures, the so-called branchial clefts. The walls between these 

 clefts are each supported by a bone, the so-called branchial arch. A 

 similar branchial arch is placed in front of the first and behind the last 

 branchial cleft. The outer margin of each branchial arch (excepting the 

 fifth ; see inferior pharyngeal bone, Section 4, c) carries two rows of 

 very fine, lancet-shaped leaflets, the branchial laminse, whilst their inner 



