DEVELOPMENT 329 



Turning to the Bony Fishes, it will be found that in its 

 essential details the development follows much the same course 

 as in the Selachians, but, as the amount of yolk available is 

 considerably less, the larva hatches out in a much less advanced 

 condition. The Salmon [Salmo) will serve to illustrate the 

 development of a typical Bony Fish with fairly large and 

 well-yolked demersal ova (Fig. 1 1 7B). As compared with that of 

 a pelagic ^gg^ the incubation period is long, and varies from 

 five weeks to more than fi\e months, being very much slower 

 when the temperature of the surrounding water is low. In this 

 connection it may be mentioned that the successful introduction 

 of Salmon and Trout into such distant countries as Australia 

 and New Zealand is made possible by the fact that the develop- 

 ment of the fertilised ova may be artificially prolonged by 

 keeping them on ice. Normally, however, the eggs of the 

 Salmon hatch out at the end of winter, and the fry or alevins, 

 about sixteen millimetres in length, remain for some time 

 hidden away in the spaces between the stones on the spawning 

 bed. They are weighed down at this stage by the large yolk-sac, 

 which is relatively smaller than that of the embryo Shark, and 

 is packed away beneath the body instead of being pendent 

 (I, II). This provides the fry with nutriment during the early 

 part of their lives, but at the end of a month or two all the 

 yolk has been absorbed and they have to fend for themselves. 

 By this time they have grown to about twenty-six millimetres 

 in length, but thereafter growth is rapid, and they normally 

 attain to three or four inches in a year, and five or six inches in 

 two years. During the first two years of their life, when they 

 live in fresh water, feeding on small crustaceans, insects, etc., 

 they are known as Parr (III), and may still be regarded as larval 

 fishes. The Parr are especially distinguished by the bluish or 

 purplish colour of the back, and by the presence of seven to 

 eleven oblong or oval spots of the same hue, the "parr-marks," 

 along the middle of each side. At the end of the two years, or a 

 little later, another change occurs, and the Parr become trans- 

 formed into Smolts. A bright silvery livery is assumed, the 

 parr-marks are obscured, and the smolts drop down the ri\ers 

 and migrate rapidly out to the open sea, where they soon 

 assume all the characters of the adult fishes. 



In those fishes with small pelagic eggs, the period of incubation, 

 although varying considerably in the diflferent species, is always 

 very much shorter. As far as our own food-fishes are concerned, 

 this period rarely exceeds two weeks, and the Anchovy {Engraulis) 



