FISHES 



43 1 



becomes deep black during the spawning season; and also our 

 Fifteen-spined Stickleback (G. spinachia), a rather larger form 

 common on the coast, and perhaps also entering some of the 

 rivers, as it does on the Continent. 



The members of the present order usually hatch out as trans- 

 parent larvae, which are very unlike their parents, and have to 

 go through a well-marked metamorphosis before they attain the 

 adult condition. A particularly instructive case is afforded by 

 the Flat- Fishes (Pleuronectidcz], of which we may take the 

 Turbot (Rhombus maximus) as an example. No one but an 

 expert would suspect that the larva (fig. 956) which hatches out 



eye A 



Fig. 956. Turbot (Rhombus maximus). A, Just-hatched larva. B, Larva eight days old: z, intestine, 

 nch, notochord; o.g., oil-globule; yk, yolk. Enlarged. 



from the floating egg of this form was destined to become a 

 flat-fish. It is a transparent creature about y^ of an inch long, 

 and, since it possesses no mouth, is incapable of feeding. The 

 food at this stage consists of the nutritive material contained 

 in the bulging yolk-sac on the under side of the body. The 

 second stage represented in the figure was taken from a larva 

 eight days after hatching. Well -developed mouth and jaws 

 were present, and the yolk was entirely used up. Somewhat 

 later on the right eye begins to move up the side of the body, 

 gradually passing over the top of the head to the left side. As 

 this curious migration goes on the upright swimming position 

 is gradually abandoned for an oblique, and finally for a hori- 

 zontal one. A swim -bladder is present. Until a length of 

 about an inch is attained the young Turbot lives at the surface, 

 regarding which Cunningham (in Marketable Marine Fishes] 

 thus speaks: " The advantage of this power and habit of 

 swimming at the surface is not difficult to discover. Even at 



