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very rare in these parts, the pelagic stage of the fry after all is so different 

 from that of the genus Pleuronectes and evidently, according to rny experi- 

 ences in aquariums, on the whole far more hardy, and particularly not afraid 

 of less saline water ; in such we often see the pelagic young ones swimming 

 about, while the young plaice at the pelagic stage must always be looked 

 for in the most saline water we have in our seas. 



On the food of the flat-fishes. 



As to the food of the plaice I shall mention here only that the grown- 

 up fishes, as it is well known, principally live on bivalves, in our seas par- 

 ticularly Abra alba, Maclra subtruncata and elliptica, Corbula nucleus, Mytilus 

 edtdis*) (of which they prefer the little ones), Solen pellucidus, Macoma bal- 

 tica, etc., but that they eat also other animals, for instance the young ones 

 of Amphidetus cordatus, more particularly however Annelida, chiefly Areni- 

 cola marina. 



The eggs of Solea vulgaris = c. 1,0265. — Corresponds to a salinity of c. 3,23%— 



3.5 %. 

 The eggs of Scomber floated at a specific gravity of 1,0265 — 1,026. Corresponds to 



a salinity of c. 3,5 %— 3,17 %. 

 Hensen (4. Bericht. 1884) states the specific gravity of the eggs of cod to be 

 1,0131 — at 17,5° = 1,72% of saline matters. 

 It will he understood after these statements of the specific gravity, why many 

 eggs of fishes are not carried into our smaller seas by the currents, but stop in 

 the northern Cattegat. — The various sorts of pelagic eggs are indeed geographi- 

 cally distributed in the seas according to their specific gravity, and this of course 

 influences the distribution of' the grown-up fish. How great influence, I wonder, 

 has this factor in the distribution of the lower animals? Cmp. my statements in 

 »Hauchs Togter« (>General Eesults«) concerning the great influence of surrounding 

 physical conditions on the distribution of the lower animals in the Cattegat. 

 Note. In >jSTordisk Tidsskrift for Fiskeris 1876, pp. 51—53 has J. Collin described 

 some plaice from the Limfjord, which had a peculiar »ring« through their mouth. 

 This ring, which is without any break, hangs loosely out of the mouth and through 

 the gill-opening on the blind side, and is so strong that the fish may be carried 

 in it without bursting it. It consists partly of various alga?, especially of the 

 genus Polysiphonia (Mutchinsia) and strips of Zost'era, partly of a tough animal 

 mucus. — He calls these plaice »ringplaice«. — 



Together with Dr. Kolderup-Rosenvinge I have once seen some of these fishes, 

 which at certain places are not rare, in the Limfjord, and by means of the micro 

 scope the tough threads in the rings were proved to be byssus-threads of 

 Mystilus edulis, which the plaice here eat in great numbers. 



I suppose that the ring is formed quite ^accidentally* by the breathing-movements 

 of the fish, when long byssus-threads are hanging out of its mouth and can reach 

 others which are hanging out of the gill-opening. 



AVli ether all rings of these » ring-flounders < consist of this matter I cannot tell. 

 .Strange to say, I never saw them except in the Limfjord. (As to the details 

 cmp. Collin). 



