546 ICHTHYOLOGY. 



to vary its specific gravity, and thus float at any desired eleva- 

 tion in the water. In appearance it varies ; sometimes, as in the 

 Sea-Porcupines or Hedge-Hog fishes, (Diodon, double toothed,) 

 and their allies, it is two-lobed ; in the Electric Eels and the Carp 

 fishes, Cyprinida,(Q\\ kuprinos, a carp,) it is divided by a trans- 

 verse partition, which in the latter allows of inter-communication, 

 through a narrow orifice. In the (Pimebdus catus,} or Com- 

 mon Cat-fish, Family Silurida, (Gr. silouros, a sheat-fish,) it is 

 divided into four cavities or compartments. In many species there 

 are closed or blind tubular processes proceeding from various parts 

 of the surface ; in others the bladder is sub-divided into as many 

 irregular cells; all this showing it to possess the rudimentary re- 

 mains of the lungs of air-breathing animals. In marine fishes the 

 bladder usually contains a gas having in it a greater proportion of 

 oxygen than of atmospheric air, while in those of fresh water, ni- 

 trogen predominates. The species which are without the air-blad- 

 der, or have it only in a rudimentary state, are generally Ground 

 Fishes, keeping close to the bottom. Sometimes the possession 

 of an air-bladder exposes fishes to danger; Gurnards, Triglida, 

 (Gr. trigla,) and Conger Eels, (Anguilla conger,) at times ap- 

 pear to distend the air-bladder so much that it loses its elas- 

 ticity or power of contraction, or as fishermen say, these fishes 

 " blow themselves," becoming unable to sink or to make their 

 escape. The Sea-Porcupine, (Plate XIV. fig.7 ; ) has the habit of 

 filling its body with air, and of fl >ating helplessly in this condition 

 at the surface ; but in the case of this fish, the air is taken, not 

 into the bladder, but into the huge stomach. 



A writer speaks " of a gentleman of his acquaintance who 

 had a Gold-fish which swam about for more than two months, 

 with its bell'y upwards, appearing perfectly healthy and lively," 

 and who attributed this change in the natural position of the fish 

 to an enlargement or defect in the air-bladder. 



Water is the well known sphere of life and motion to fishes. 

 The Flying fishes, Exocatus volitans, (Plate XIV. fig. 3,) and 

 some of the Gurnards can indeed raise themselves into the air and 

 keep their position there for a few seconds ; some of the Frog-fishes, 

 Lophida, (Gr. lophos, a neck or crest,) and Eels, AnguittidcB^ (Lat. 

 anguilla, an eel,) can crawl upon the exposed mud or sand in the in- 

 terval occurring between the ebb and flow of the tide ; and the An- 

 abassidae, or Climbing Perches leave the water in order to obtain 

 food ; but these are only deviations from a general rule. Some in- 

 habit fresh water only ; some only the sea ; others can exist in both, 

 either by periodical migration, or at pleasure. Near the city of 

 Bristol, (Eng.,) the Eels are known annually to ascend the trees 



