8 NEW SOUTH WALES 



advance of this, witli a free margin like a second gill ; this is called the 

 pre-opercidum. It varies considerably in shape, often having a toothed 

 edge or spines, and so becomes useful as a feature in classification. The 

 term opercuhim is, as already said, only applied to the hind margin of the 

 gill-cover, which is divided into movable segments. The under one is 

 called the sub-ojyerculum, and the segment above is called the inter-oper- 

 culum ; they are separate bones. These bones are sometimes rudimen- 

 tary and sometimes absent. All of them are frequently referred to 

 in descriptions of fishes in scientific works. 



Everyone must have noticed along the sides of most bony fishes a line 

 something like a division between the belly and back. It is sometimes 

 straight, but more often is curved in the most varied manner. This is 

 called the lateral line, and is caused by a series of perforated scales 

 through which mucus is especially secreted, though no doubt all the sur- 

 face of a fish secretes the same fluid to some extent. This perforated 

 line is provided with abundant nerves, and is called the muciferous 

 system. Some fishes have many lateral lines, and our coast and river 

 mullets have none at all. 



All fishes are of distinct sex. The females in the majority of instances 

 are oviparous. A few bear the young alive ; generally the eggs are 

 deposited by the female and are afterwards fertilized by the male. In 

 the males, lying along the intestines, there are two soft bodies called the 

 milt. In certain seasons these contain a milky secretion, which is the 

 fertilizing fluid. This is deposited on the eggs by the male, or merely 

 injected into the water. A very small quantity of this fluid is sufiicient 

 for the impregnation of an immense number of ova, and it is owing to 

 this circumstance that the artificial impregnation and hatching of fishes 

 is easily practised and immense numbers of fish preserved. 



In the same relative position as the milt (soft i"oe) in the male are 

 found the bodies called the hard roe in the female. This is a mass of 

 unimpregnated eggs. 



In their young states fishes differ so much from the forms they assume 

 in full growth that they have been very frequently mistaken for distinct 

 species. Though a number of such mistakes have been rectified of late 

 years, there are doubtless still some received species which ai-e only the 

 young of other forms. Until all the stages of growth are known these 

 errors will not be detected. 



Having now dealt with all the most necessary elements of scientific 

 classification, we can apply them to the general divisions which are 

 made. Let it be premised, however, that those first entering upon this 

 study must not be disheartened if they find it difficult to identify with 

 certainty some of the details. Though perhaps a little out of place in 

 such a work as this, it may be well to suggest a method l)y which the 

 details of scientific description may be mastered. Instead of taking an 

 unknown fish and trying to make it out by the aid of a catalogue, such 

 as Giinther's or Macleay's, let the student compare a fish, the scientific 

 name of which he is sure of, with the description given of it in these 

 works. When this has been done in the case of ten or a dozen species of 

 diff"erent families, the work of identification will be comparatively easy. 

 All who are in reach of the Museum in Sydney, or any of our colonial 



