ORGANS OF RESPIRATION. 247 



branchial arches below are seldom wanting, as in Muraenophis, 

 Syngnathus, and Lophius. The Cartilaginous Fishes possess simi- 

 lar branchial arches, only they are cartilaginous, and five instead of 

 four in number, of which, however, the most posterior corresponds 

 to a pharyngeal maxilla ; the arches abut, in the Sharks, against 

 some intermediate cartilaginous pieces, and consist themselves of 

 several segments ; the branchial arches in the Rays resemble in 

 general those of the Sharks, and are united inferiorly by one or two 

 very broad sternoidal cartilaginous plates. In most Osseous Fishes 

 the branchial arches are situated beneath the skull, but in Fishes of 

 the Eel kind, e. g. Muraena, Mursenophis, they are situated further 

 back beneath the first vertebrae ; in the Rays and Sharks they are 

 united still further back to the commencement of the vertebral col- 

 umn. In the Cyclostomi we find a very peculiar branchial skeleton, 

 formed of narrow arch-shaped cartilaginous rays that surround the 

 gills. 



The gills, which in the earlier stages of development of Fishes 

 were freely exposed and unprotected, are always concealed at a later 

 period of existence beneath the skin, and protected also by special 

 covers or opercula ; these are most evolved in the Acanthoptery- 

 gians, and form for the most part a large bony apparatus, seldom in- 

 ferior in size to the branchial, but which is superseded in the Chon- 

 dropterygians by another contrivance. There are most frequently 

 four, or if we regard, with many anatomists, the praeoperculum as 

 the quadratal bone, three bony pieces, composing the opercula of the 

 gills. The Praoperculum is of a semilunar form, and bounds pos- 

 teriorly the series of bones belonging to the articulating or quadratal 

 portion of the temporal bone. To this succeeds, in the direction up- 

 ward and backward, the true Operculum, a flat and more or less 

 quadrangular bone, the largest in size, and presenting upward and 

 forward a socket that articulates freely with a capsular head upon 

 the uppermost quadratal bone. Upon the posterior and inferior 

 edge of the operculum is situated the Postoperculum, and between 

 this and the praeoperculum, behind the lower jaw, is placed the 

 Interoperculum. This opercular apparatus serves as a valve, where- 

 by the fissure leading from without into the branchial cavity, and 

 placed between the posterior edge of the operculum and the ante- 

 rior girdle of the pectoral fins, may be opened or shut. The position, 

 but more especially the size of the opercular bones, varies ; one or 

 other of them is frequently ill-developed, as in the Eel family. 

 Among the true Cartilaginous Fishes we find, instead of this appa- 



