60 COMPARATIVE ANATOMY. 



teriorly by the alimentary canal. It is thus at once a bran- 

 chial chamber and a pharynx ; hence the name of the class, 

 including the Amphioxus Pharyngobranchiata. The pro- 

 cess of respiration would appear to be aided by a number of 

 very vascular processes of mucous membrane produced from 

 the parietes of the chamber at its anterior portion. In mar- 

 sipobranchiates the branchiae are enclosed within pouches 

 (hence the name given to the class) upon the side of the 

 body, each of which has a distinct internal and external ori- 

 fice.* In Selachia and osseous fishes, the arches vary from 

 three to six in number; the branchial interspaces opening 

 externally by a number of distinct apertures, as in the former, 

 or terminating in common, and protected by portions of skel- 

 eton (operculum), as in the latter. In Selachia each branchia 

 assumes somewhat a ribbon-like form (hence the name Elas- 

 mobranchiata). In osseous fishes it may assume the form of 

 a lobe or crest, as in Hippocampus (sea-horse), or that of a 

 comb, as in all others. 



False gills are vascular appendages to gill covers of certain 

 Selachia. 



Swim bladder. This is a simple or sacculated hollow viscus, 

 present in many fishes ; absent in Scomber (mackerel). It is 

 oblong, and ordinarily furnished anteriorly with two caeca. 

 Possessed of a smooth fibrous covering, it is lined with a 

 glistening membrane, exceedingly rich in gelatine (ichthyo- 

 colla, q. v.), and at one portion of the surface is remarkably 

 vascular, constituting the rete mirabile. The swim bladder, as 

 a rule, is situated directly beneath the vertebral column, 

 rarely within a vertical fold of intestine, and thus separated 

 from contact with the abdominal walls, as in Campostoma. 



* To understand the relation between these and the gills of ordinary 

 fishes, we must suppose each compressed sac to be split through its plane, 

 and each half to be glued by its outer smooth side to an intermediate sep- 

 tum, which would then support the opposite halves of two distinct sacs ; if 

 then these vascular surfaces be prolonged in gill-like structure, and an inter- 

 mediate basis of support be given, we have the branchial arch of a Selachian, 

 which is the homologue, not of a single gill sac, but of the contiguous halves 

 of two distinct gill sacs. (Carpenter.) 



