ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 705 



its mode of life, for it lias two pedal lobes which become firmly attached 

 around the orifice of the shell. Tho Actinian would appear to seize 

 and strike smaller mariue animals by its stinging cells, and these 

 would thus come into the area of the sedentary crustacean. Like other 

 writers on this subject at the present time, the author directs his 

 notice to the yellow cells found in the Kadiolaria. 



Pelagic Fauna of Fresh-water Lakes.* — Professor F. A. Forel 

 considers that Entomostraca alone show the peculiar character of 

 pelagic animals, the pelagic fauna in its general features being similar 

 in all the countries and lakes of Europe yet investigated, though 

 seldom represented in any one lake by all the animals of the fauna. 



The characters common to the animals of the pelagic region are 

 due to their mode of life. Tbey must swim incessantly, and there- 

 fore, instead of any organ of adhesion, they have a highly developed 

 natatory apparatus ; they are sluggish, and escape their enemies by 

 their nearly perfect transparency, which may be regarded as a 

 mimicry acquired by natural selection, only those having held their 

 own which are as transparent as the medium in which they live. 

 They perform daily migrations, during the night swimming at the 

 surface, and in the day descending into the depths. 



As to the origin of the pelagic fauna, the author decides that 

 certainly the palustrine or fluviatile Entomostraca have not become 

 transformed in each lake into pelagic species or varieties. The 

 almost complete identity of the European pelagic Entomostraca shows 

 a common origin and distribution, and he believes that we must find 

 the cause of the differentiation of the pelagic fauna in the combination 

 of two different phenomena — the daily migrations of the Entomos- 

 traca, and the regular local winds of the great lakes. On the borders 

 of great masses of water two winds prevail, one blows at night from 

 the land to the water, the other by day from the water to the land. 

 The nocturnal animals of the shore-region which swim at night at the 

 surface are at this time driven towards the middle of the lake by the 

 surface-current of the land wind, sink during the day (being driven 

 away by the light into the deep water) and thus escape the surface- 

 current of the lake-wind, which would otherwise have carried them 

 again to the shore. Constantly driven further every night, they 

 remain confined to the pelagic region, as they are not carried back 

 again during the day. Thus a differentiation takes place by natural 

 selection, until at last, after a certain number of generations, there 

 remain only the wonderfully transparent and almost exclusively 

 swimming animals which we know. When this differentiation has 

 once taken place, the pelagic species is conveyed by the migratory 

 water-birds from one country to another, and from one lake into 

 another, where it reproduces its kind if the conditions of the existence 

 of the medium are favourable. In this way we may find the pelagic 

 Entomostraca in lakes which are too small to possess the alternation 

 of winds, the animals having been differentiated by the action of the 

 winds in other larger lakes. 



* Biol. Centralbl., ii. (1882) pp. 209 305. Ann. and Mag. Nat. Hist., x. 

 (1882) pp. 320-5. 



Ser. 2.— Vol. II. 3 F 



