12 CLASSIFICATION OF FISHES. 



no metamorphosis. These gills, or branchia, as every 

 one knows, are composed of certain semicircular arches, 

 fringed, as it were, with thin fleshy processes, resembling 

 little leaves, or laminae, having innumerable blood-ves- 

 sels : the water taken in by the mouth, again escapes 

 between the openings of the branchia, which are pro- 

 tected externally by certain bony plates united together, 

 yet generally movable, which are called the 0}jercul<E, 

 or gill covers : this apparatus for breathing is variously 

 modified, but never lost, so that it is the primary cha- 

 racter by which fishes are at once distinguished from 

 reptiles and amphibians. 



(12.) The operculum, or gill cover, just mentioned, 

 is articulated on the os tympaiii, and is moved upon a 

 piece called the pre-operculum : it is composed of three 

 bony plates, termed the operculum, the suh-operculum, 

 and the inter-operculum ; the modifications of which 

 are sometimes of much use in determining natural 

 affinities. In many groups, however (as in the eels), 

 the operculum is so entirely covered by the common 

 skin as not to be visible but upon dissection ; and 

 among the cartilaginous and some other fishes, the oper- 

 culum is entirely wanting. 



(13.) The SKELETON of all fishes, except such as 

 are lowest in the series, present a vertebrated column, 

 and other internal bones ; but the structure of these 

 bones is very different, and, as may be expected, indi- 

 cate the primary divisions of the whole class. In the 

 most perfectly formed fishes, the bones are completely 

 osseous, and generally of great hardness : in another 

 large division, they are fibro-cartilaginous — that is to 

 say, the base or heart of the bones is of gristle, or is 

 cartilaginous, mixed only with fibres or layers of phos- 

 phate of lime, so that the texture is never so hard as in 

 the osseous groups just mentioned : some of these semi- 

 cartilaginous genera, indeed, have their bones quite 

 soft, and thus lead to the third group, or ti*uly car- 

 tilaginous fishes, which, hke the sharks and skates, 

 have their skeleton composed of gristle or cartilage 



