102 



MARKETABLE BRITISH MARINE FISHES 



in the skin and other parts, and which begin to appear some 

 time before hatching. 



The time occupied in the development of the embryo in the 

 egg differs in different species, and in the same species cHffers 

 according to the temperature of the water in which the eggs are 

 contained, being quicker when the water is warmer, slower when 

 it is colder. It varies from three or four days to as many weeks 

 in the case of buoyant eggs, but is longer in the case of some of 

 the larger attached eggs. 



The larvae hatched from buoyant eggs are very small : one of 

 the largest, that of the plaice, is only i- inch in length. The 

 larvae hatched from attached, adhesive eggs are usually more 

 developed, and differ among themselves in size and degree 

 of development. Such eggs are in nearly all cases larger than 

 buoyant eggs, they have a larger yolk and a stronger egg- 

 membrane : the larva therefore remains within the latter for a 

 longer time, and does not leave it until it has reached a more 

 advanced condition. Usually the mouth is open and the jaws are 

 somewhat developed, and there is red blood in the veins. Among 

 the most advanced fish at the time of hatching are the salmon 

 and trout, in which the longitudinal fin-membrane is already 

 separated into portions indicating the permanent fins. Fig. 55 



Fig. 55. — Newly hatched young (larva) of the Common Black Goby. 



shows the condition of the young of the black goby when first 

 hatched. 



For some days the fish larva hatched from buoyant eggs, such 

 as the larva of the plaice or cod, continues to develop without 

 taking food, being nourished by the mass of yolk it still possesses. 

 The quantity of yolk continually diminishes ; the mouth appears 

 first as an opening on the lower side of the head, as it is in the 

 full-grown dog-fish, and then is carried to the end of the snout 

 by the growth of the jaws. The pigment in the body increases, 



