VI. Biology. 1. Dispersal of Anim.als. 61 



Cavernicolous Fauna. — The entirely dark, subterranean caves 

 which occur in mountains of different countries, and which 

 are also present beneath the water, harbour a small but peculiar 

 fauna. Most of the animals living in these places, in contrast to- 

 their relatives of the daylight, exhibit very degenerate optic 

 organs, or these are even entirely wanting. Frequently the skin 

 is destitute of pigment : the blind, pale Proteus of the Carniola caves, 

 may be taken as a typical cave animal, and some Fish, certain 

 Crustacea, Insecta, Spiders, also belong to this fauna. All cave 

 animals, however, are not blind; some have retained their eyes, and 

 are consequently less suited to life in the dark. 



Those animals, wliicli biu-row in the earth, and wHch come to tlie surface 

 seldom or only on dark nights, are like the cave forms ; then- eyes, too, may 

 be more or less degenerate (Moles). 



The rich animal life of the deep sea, where no daylight 

 penetrates, resembles the cave fauna. The deep sea animals are often 

 almost destitute of pigment, and have, frequently, very degenerate 

 eyes, or indeed none, even when belonging to groups, the other 

 members of which have these organs well-developed (some deep sea 

 Crustacea and' Fish) . Other deep sea forms are, however, furnished 

 with well-developed eyes, e.g., the majority of the fish, which are 

 considered abysmal.* Many of these Fish are phosphorescent. Among 

 the abysmal animals there are many which have only remote relations 

 elsewhere — Stalked Crinoids, peculiar Eohinoids and Crustacea,, 

 Hexactinellids, and others, so that this fauna is strikingly peculiar. 



Pelagic Eauua, — Still more extraordinary are the animals 

 living in the open sea, at a considerable distance from land, the so- 

 called Pelagic Fauna. This comprises first, a number of groups, 

 which either never, or only exceptionally come close to shore, and 

 which generally do not occur elsewhere: the Radiolaria and other 

 groups of the Protozoa, the Siphonophora, Euphausidee, Pteropoda, 

 Heteropoda, and Salpidse : secondly, a multitude of forms whose near 

 relatives live closer to land : lastly, an immense number of larvae, 

 belonging to forms which live in the adult state at the bottom of 

 the sea. But what specially characterises the pelagic fauna is not so 

 much the number of groups peculiar to it, as the fact that in virtue 

 of their life in the surface waters, all the members of these different 

 groups have certain features in common which are, however, less well 

 marked in some than in others. There is, in particular, an evident 

 tendency towards such a modification of the animal as may enable it 

 to float with the greatest facility. This is attained, in some cases, 

 by the tissues absorbing such considerable quantities of water that the 

 specific gravity is only slightly greater than that of the sea ; such 

 animals have a jelly-like appearance (Medusae, many Pteropoda and 



* It is possible, however, that many of these live nearer the surface, and were 

 caught in the net as it was drawn up from the bottom. 



