4 FISHES OF AUSTRALIA. 



The branchiostegal membrane is that thin mem- 

 brane attached to, and just beneath, the posterior portion 

 of the opercle, and supported by cartilaginous or bony 

 rays, known as the branchlostegal rays. 



The teeth of fishes are usually conical and pointed, 

 though frequently some are incisor- or molar-like ; an 

 instance in which the latter are possessed, being that of 

 the common Black Bream. Other modifications in shape 

 are also to be found, such as in the case of the common 

 Blackfish, where the teeth are comb-like or tricuspid, and 

 also in the Toadfishes, in which the teeth are welded 

 together, so as to form hard, chisel-like plates. 



Many of my readers, particularly those who keep 

 aquaria, may have often noticed that certain fishes are 

 able, without any apparent effort, to rise or sink in the 

 water, at will; and perhaps they have wondered what 

 means were employed in doing this. The answer is, that 

 the fish is able to perform this seemingly impossible feat, 

 solely through the possession of what is termed the air- 

 bladder (known also as the "swim-bladder" or "sound"). 

 The air-bladder which is one of the most characteristic 

 organs of fishes, is a hollow sac, containing gas, 

 situated in the abdominal cavity, immediately beneath the 

 spinal column. By being compressed or dilated, it in- 

 fluences the specific gravity of the fish; thus enabling it 

 in some instances to rise or sink in the water, without the 

 aid of the fins at all ; while in many other species, in which 

 it has not undergone such a degree of development, it is 

 still of very great assistance. In some fishes, only the 

 rudiments of an air-bladder are to be found; while in 

 others, there is no trace of it at all. In the Lung-Fishes, 

 it assumes the function of a simple kind of lung ; enabling 

 them to breathe air. In the fishes of the Jewfish family, 

 and also in the family of the Threadfins or Tassel-Fishes, 

 the air-bladder is very large and important. 



The eyes of fishes are usually rather large for the size 

 of the body (in some instances they are enormous), but 

 in certain cases they are either very small or else quite 

 rudimentary and hidden by the skin. A familiar instance 

 of a large-eyed fish is the Nannygai, while blind fishes 

 are to be found either in caves or at great ocean depths. 



