XIL] THE EXCRETORY ORGAXS. 477 



The branchial arches of osseous Fishes are made up of 

 parts to which special names have been applied. They are 

 attached to a median ventral series of pieces termed bast- 

 branchials. The lower piece of each lateral arch is called 



FIG. 402. FIRST THRE^: BRANCHIAL ARCHES FROM THE LEFT SIDE OF A PERCH. 

 On the outer (convex) side of each branchial arch the series of closely-set gill 

 filaments (or leaflets or lamellae) are seen to be attached. On the inner (con- 

 cave) side of the first branch! il arch are the series of elongated processes 

 (supporting minute denticles) which help to prevent particles of food, or other 

 foreign bodies, passing from the mouth to the giil chamber. 



a hypo-branchial', to this succeeds a cerato-branchia.1, then 

 an epi-branchial) and finally, at the summit, a pharyngo- 

 branchial. These solid, supporting parts are not constantly 

 or equally developed in all the branchial arches, as is shown 

 by the annexed woodcut of these arches as they exist in the 

 Perch (Fig. 403). 



The partitions between the gills may extend from this 

 solid internal support, outwards to the skin as in the Sharks 

 and Rays. In such case each gill-chamber opens separately 

 on the side of the body. 



The partitions may, however, extend but a very little way 

 from such solid internal support, outwards to the skin, while 

 the vascular gill-membrane remains elongated, and conse- 

 quently hangs freely from the outer side of each branchial 

 arch. This latter condition is that which obtains in ordinary 

 osseous Fishes, e.g. Perch, Carp, Cod, &c Here we find but 

 one large common chamber in which the filamentary gills 

 float, but this chamber is protected by the extension back- 

 wards of a fold of skin (the operculuni) supported internally 

 by the opercular bones ; and thus there comes to be in 

 ordinary Fishes but a single large gill-opening on each side 

 behind the head. 



