2 FISHES OF AUSTRALIA. 



opening; while the tail is generally said to begin at the 

 vent. There are numerous exceptions to the latter, how- 

 ever, particularly among the "flat-fishes" of the family 

 Pleuronectidce, such as our Soles and Flounders, in which 

 the vent is situated far forward and near to the head. 



If one examines a fish, he will find at the back of the 

 head, on each side, a kind of moveable flap covering the 

 gills. This is the gill-cover or opercle, which is com- 

 posed of three bony plates, styled the opercle, the sub- 

 opercle and the inter-opercle; the modifications of which, 

 are sometimes of considerable value in determining the 

 natural affinities of fishes. In some groups, such as the 

 Eels and the Dragonets, the opercle is so entirely covered 

 by the skin, as not to be visible, excepting after dissec- 

 tion; while in the cartilaginous fishes (Sharks, Rays, &c.) 

 it is entirely absent. 



The gills, or, as they are termed, the branchiae, are 

 fine hair-like or leaf-like projections, filled with innumer- 

 able bloodvessels, and usually supported on the outer 

 curves of cartilaginous or bony arches, known as gill- 

 arches. In the true fishes, the normal number of gills 

 on each side is four.* Along the inner edges of the gill- 

 arches, are often to be found, hard, bony appendages or 

 projections, which are subject to great variation in dif- 

 ferent species. These are known as the gill-rakers. 

 The gill-membranes are those which usually serve to 

 attach the gill-covers to the isthmus, which is that thick, 

 fleshy projection between the gill openings. Occasion- 

 ally are to be found on the inner side of the opercle, near 

 its junction with the preopercle, small or imperfectly- 

 developed gills, which are known as pseudobranchice. In 

 the process of breathing, the water is constantly taken in 

 by the mouth and passed out again between the gills ; in 

 its passage aerating the blood, as it circulates through the 

 gill- filaments. This is the habit which has given rise to 

 the saying, "drink like a fish," which, of course, has no 

 foundation, as the fish does not really drink the water. 



* Sharks and Rays have usually five, and occasionally (in the case 

 of the " One-firmed " Sharks) even six or seven. 



