of the Fishery Board for Scotland. 



183 



were easily distinguished from the rest by the light colouration of their 

 food. 



It was first now that the larvae were able to cope with Evadne, which 

 soon became the principal food. It was not, however, so easily deter- 

 mined what crustacean those remains originated from which I found in 

 the stomachs. I have figured some forms of them (fig. 16) just as they 

 appeared after being spread out under a cover-glass. Amongst the 

 bundles of small limbs and spines there were invariably patches of a 

 black substance, which apparently had not been influenced by the 

 digestive process in the fish. Through experiments with Evadne I 

 found that its large ' black eye ' took such an appearance, and was further 

 satisfied in the identification by closer comparison of the limbs. 



The larger part of the body of Evadne is almost transparent, but 

 denser than the water, so that the light is refracted when passing it ; this 

 gives the crustacean a great resemblance to a glass-bowl or an air-bubble ; 

 and it is here of interest to remember my observation on the plaice larvae 

 in 1894 (Twelfth Annual Report, Fishery Board, Part III., p. 216). 



Air had been forced into the water artificially, and minute bubbles that 

 had been retained in the water were swallowed by the young fishes to 

 such an extent that the accumulated air in the stomach ultimately killed 

 such individuals. The same air-bubble-like part of Evadne seems to be 

 readily digested in the stomachs of the post-larval plaice, as I always 

 failed to see remains of it amongst the food. 



Another form of molluscs was also common now amongst the food, 

 namely, larval Scrobicularia alba (fig. 17), of which four to six specimens 

 were sometimes found in one stomach. Of additional forms of crusta- 

 ceans, I observed a copepoda, Calanus finmarchims. 



On the 30th day an average specimen measures 9*40 mm. in length and 

 1*91 mm. in breadth. The increase in size during this period was there- 

 fore greater than in any previous period ; and the general development of 

 the larvae had also proceeded more rapidly than before. 



Most noticeable was the increased number of caudal fin-rays, which had 

 also appeared on the dorsal part of the now much more heterocercal-tail. 

 About one dozen spinal processes had developed ventrally on the posterior 

 half of the spine, and a few more on the dorsal side. No rays were yet 

 seen in the marginal fins. The yellow pigment was chiefly confined to a 

 row or line following the base of the dorsal marginal fins, while the latter 

 had got a broad line of black stellate pigment along the edge on its pos- 

 terior half. The lateral line was now also prominently marked with 

 black pigment, while the latter as before was richest on the ventral side 

 of the body and on the ventral fin. A few brick-red pigment corpuscles 

 had appeared along the base of the marginal fins. 



Zlst to day. 



The fry were now mostly all feeding at the bottom amongst the con- 

 stantly increasing quantity of living and dead organisms, and the contents 

 of the stomachs were coloured brown or black. It is difficult to say 

 whether the larvae took any dead food at all, or adhered to such alone 

 that was alive. There is reason to believe that living food is the most 

 natural for them, or at least suitable, judging from the smartness and 

 ability exhibited on the part of the fry in pursuing quickly moving forms, and 

 considering circumstances in the sea, where the post-larval stage is supposed 

 to be spent in the higher levels, at least away from the bottom, dead 

 forms of crustaceans, etc., will seldom be met with in nature. But while 

 living forms appear the most natural, I would consider it quite possible 



