CLASS-BOOK OF ZOOLOGY. 10V 



consists, as in the preceding classes, of vertebrae, of 

 which each species has a determinate number. 



§ 451. Most Fishes are provided with a swimming- 

 bladder, which they can fill with air, or contract, in 

 order to facilitate their ascent or descent in the water. 



§ 452. This swimming-bladder is a membranous air- 

 vessel of an oval form, situated along the back-bone, 

 and sometimes divided into two or more prolongated lobes, 

 which have a communication with the stomach by a small 

 channel, called the Pneumatic channel. 



§ 453. It cannot be doubted, that it is by means of the 

 the swimming-bladder that Fishes effect their ascent or 

 descent in the water ; for if we pierce, with a fine needle, 

 the swimming-bladder of a living fish, it descends to the 

 bottom, and is not able to ascend. Where artificial fish- 

 ponds are near together, and each contains a different 

 species of fish, this expedient is sometimes resorted to, 

 in order to prevent them from passing from one pond 

 into another. 



§ 454. The eggs of which the fish-roe is composed 

 are very small, and their number in some species is so 

 immense, that if the greater part of them were not the 

 prey of other fishes, the ocean would soon be found too 

 small for its inhabitants. There have been found in one 

 Herring 50,000, in a Carp 200,000, in a Flounder one 

 million, and in a Cod-fish nine millions of eggs. 



§ 455. The fish-eggs are hatched in the water by the 

 caloric of the atmosphere. The time required depends 

 upon the species. Some require only seven days, others 

 a longer time, and some fifty days, which is the longest. 

 The embryo is developed in the same manner as that of 

 birds and reptiles. 



§ 456. The use of fish as food is universal. Thou- 

 sands of families are supported by fishing ; and in some 

 parts of the world the Shad, Mackerel, and Cod fisheries 

 are a principal branch of industry. 



§ 457. The Icliihyopliagi (fish-eaters) on the shores of 

 the Red Sea in Arabia, the Kamschatkadales, and some 

 other people, make a kind of bread, by drying fish in 

 the sun, and converting them into flour. 



