472 MANUAL OF ZOOLOGY. 



structure of the branchiae differ a good deal in the different 

 orders of fishes, and these modifications will be noticed sub- 

 sequently. In the meanwhile it will be sufficient to give a 

 brief description of the branchial apparatus in one of the bony 

 fishes. In such a fish, the branchiae are connected with the 

 hyoid arch, and are situated in two special chambers, situated 

 one on each side of the neck. The branchiae are carried upon 

 the outer convex sides of what have been already described as 

 the " branchial arches ; " that is to say, upon a series of bony 

 arches (figs. 250 and 256) which are connected with the 

 hyoid arch inferiorly, and are united above with the base of 

 the skull. The internal concave sides of the branchial arches 

 are usually furnished with a series of processes, constituting 

 a kind of fringe, the function of which is to prevent foreign 

 substances finding their way amongst the branchiae, and thus 



Fig. 256. Gills and heart of the Perch exposed by the removal of the gill-cover on the 

 left side, a First of the four bony arches which carry the gills (b b) ; b' The lower 

 edges of the gills on the right side ; k Heart. (After Van der Hoeven.) 



interfering with the proper action of the respiratory organs. 

 The branchiae, themselves, usually have the form of a double 

 series of cartilaginous leaflets or laminae. The branchial la- 

 minae are flat, elongated, and pointed in shape, and they are 

 covered with a highly vascular mucous membrane, in which 

 the branchial capillaries ramify. The blood circulates through 



, the branchial laminae, and is here subjected to the action of 

 aerated water, whereby it is oxygenated. The water is con- 



1 stantly taken in at the mouth by a movement analogous to 

 swallowing, and it gains admission to the branchial chambers 

 by means of a series of clefts or slits, the " branchial fissures," 

 which are situated on both sides of the pharynx. Having 

 passed over the gills, the deoxygenated water makes its escape 



