THE SWIMMING-BLADDER. 251 



rence. As a rule, the swimming-bladder contains only a single 

 cavity ; frequently, however, two, one placed behind the other, and 

 separated by a deep constriction, as in the Carps and many Salmons ; 

 in Blennius Phycis we find three, and in Polypterus two such divis- 

 ions ; in Trigla hirundo there are also three, but they are arranged 

 side by side. Pimelodus filamentosus is furnished with two com- 

 pletely-separated swimming-bladders lying one behind the other. 

 The swimming-bladder is sometimes provided with caecal pouches, 

 varying in size, form, and length, e. g., in some species of Gadus ; but 

 they are most distinct in the family of Sciaenidae, as in S. umbra, in 

 Johnius, Pogonius, Corvina, and Otolithus, where the appendages 

 are divided in a digitate manner. It is rare for cells to be developed 

 upon the internal surface of the swimming-bladder, in which case 

 that organ greatly resembles the lung of an Amphibian ; this is ex- 

 emplified in many Erythrini, and in several genera of Siluroidae, e. g., 

 Bagrus and Arius, where the swimming-bladder is divided into several 

 intercommunicating chambers by imperfect partitions ; in Platys- 

 toma we meet, in addition, with a pair of cellular wings or appenda- 

 ges. The very long swimming-bladder of Lepisosteus, which ex- 

 tends from the pharynx to the anus, is provided superiorly with 

 two blind appendages, but is in other respects simple, except that a 

 part of its internal cavity is furnished with smaller cavities or 

 pouches upon the floor of which the mucous membrane forms a net- 

 work of parietal cells. In many Fishes the swimming-bladder is 

 peculiarly situated. Thus in Cobitis fossilis it is completely en- 

 closed in a bony capsule formed by the transverse processes of the 

 third cervical vertebra. In Heterobranchus it is situated trans- 

 versely within a conical bony capsule opening by a fissure inferiorly, 

 and formed by an expansion of the transverse processes. 



The swimming-bladder is either entirely closed, or provided with 

 a tube, as in most of the Ventrales, while in the Pectorales and Jug- 

 ulares this is usually wanting. The tube consists of the same 

 coats as the bladder, and is either short or long, narrow and tortu- 

 ous ; it runs in the direction downward and forward, and perforates 

 the cesophagus in different situations, and in some cases the com- 

 mencement, or what is more rare, the bottom of the stomach/ In 

 the genus Salmo the tube arises from the anterior extremity of the 

 bladder, or from its second division, as in Cyprinus, where it is very 

 narrow and tortuous ; in the Pike, however, it is short and wide, 

 and in Clupea it enters the base of the stomach. This opening of 

 communication of the swimming-bladder with the intestine has been 



