LIFE HISTORIES 345 



The eggs of Teleosteans are always small but some are larger 

 than others; the largest are found among those which are heavier 

 than the water, the smallest among those which are pelagic. 

 Those of Gymnarchus one of the African soft-finned Mormyridae 

 are 10 millimetres in diameter, and some of those of the cat- 

 fishes (Siluridee) which carry them in their mouths are as large as 

 this ; those of the salmon are about 5 millimetres. The largest 

 of the eggs of marine fishes are those of the cat-fish Anarrhichas 

 lupus, one of the Blennies, which develop at the bottom in deep 

 water ; these are 6 millimetres in diameter (a millimetre, de- 

 noted by mm., is -~ in.). The larger the eggs the more ad- 

 vanced in development are the larvae at the time of hatching ; 

 in pelagic eggs the larva when hatched is often without pigment 

 in the eyes, and the mouth is not open. In some cases, as in 

 the angler, the larva is provided with long appendages, formed 

 usually by the elongation of certain fin-rays, and these append- 

 ages disappear in the adult. (Fig. 27.) In this case the differ- 

 ence between the larval stages and the adult is very great in 

 consequence of the corresponding difference in habits, the larva 

 being pelagic and the adult living a sedentary life on the bottom. 

 The most extraordinary metamorphosis among all fishes is that 

 of the Pleuronectidae. In these the larva is similar to those of 

 thousands of other species hatched from pelagic eggs and is 

 perfectly symmetrical, always swimming upright in the water 

 and having an eye on each side, and also pigment on both sides. 

 (Fig. 28, A, B.) As the fin-rays and skeleton develop the eye 

 of one side changes its position and passes first to the edge of 

 the head, and finally to the opposite side, so that both eyes are 

 on one side as in the adult (Fig. 28, C, D). In some species 

 the upper side, possessing eyes and colour, is the right as in 

 plaice, flounder and sole, in others it is the left as in brill and 

 turbot. At the same time the colour disappears from the 

 lower side and the dorsal fin extends forwards towards the end 

 of the snout. The transformation is completed while the fish 

 is still very small ; in the plaice and flounder the completely 

 transformed young fish is about \ in. to f in. long ; the turbot 

 and brill reach a length of one inch or more before their meta- 

 morphosis is complete, and they may be often seen at that size 

 swimming at the surface in harbours. In one form of larva 

 originally called Plagusia but now known to belong to the 



