Page:Mrs Beeton's Book of Household Management.djvu/335

 CHAPTER XI.

In Natural History Fish form the lowest of the five classes into which the Vertebrata, or animals having a backbone, are divided. They may be broadly described as vertebrate animals living in water, and breathing the air contained in it by means of gills, which supply the place of lungs. Fish are furnished with a heart, which, except in the mud-fish consists of a single auricle and ventricle, and fins, which take the place of the limbs of animals higher in the scale of being. The blood-corpuscles are mostly red, and the blood is termed "cold," from the circumstance that its temperature is very little, if any, higher than that of the surrounding water.

The adaptability of the fish to the element in which it lives is seen in the body. In most cases the external shape offers the least possible friction in swimming, thus securing rapid locomotion. The body is, in general, slender, gradually diminishing towards each of its extremities, while it is also rounded on the sides, roughly resembling the lower part of a ship's hull, and enabling the fish, like the vessel, to penetrate and divide the resisting fluid with comparative ease. Owing to the great flexibility of the body in the water, the fish can with ease migrate thousands of miles in a season.

The Principal Organs employed by Fish to accelerate motion are their air-bladder, fins, and tail. The air-bladder, or "sound," is automatically in origin the same as a lung, but it does not perform the function of that organ. Its use is to enable the fish to rise or sink in the water. The air-bladder is a sac or bag filled with gas, chiefly oxygen in the case of sea-fish, and nitrogen in fresh-water species. When a fish wishes to sink, it compresses the muscles of the abdomen and ejects is from the air-bladder, thus increasing the weight of the body. When it desires to ascend the abdominal muscles are relaxed. This causes the air-bladder to fill, and the fish then rises to the surface. 289