180 THE DISTRIBUTION OF 



depths. Thus the members of the genus Macrurus, 

 believed to be true abyssal, that is groimd-living, forms, 

 have apparently such eggs and larvae. The abyssal 

 hermit crabs, also, have bathypelagic larvae, just as 

 the Uttoral forms have larvae which form part of the 

 surface plankton. 



(3) The abyssal region of the oceans is contrasted 

 with both the other regions in that its waters must 

 be permanently calm. It is in harmony with this 

 calm that we find, as already stated, some deep-sea 

 sponges which have no attachment to the substratum, 

 upon which they seem to lie loosely. The pressure 

 must be enormous, for the animals of the depths sup- 

 port a weight of several miles of sea-water upon their 

 bodies. Though there are no plants, oxygen seems to 

 be abundant. But this is a consequence of the well- 

 developed circulation of the great oceans, and is not 

 a feature of enclosed seas. Thus the Black Sea has no 

 animals in its depths, and the waters there are impreg- 

 nated with sulphuretted hydrogen, and contain but 

 Uttle oxygen. In the Mediterranean the conditions are 

 less extreme, but the deep-sea fauna is scanty. Light 

 is, of course, entirely absent in the depths. The water 

 is permanently cold. The basal food-supply must 

 consist of the dead animals of the upper zone, which 

 fall downwards through the water as they die. In 

 consequence we find that many of the abyssal forms 

 are mud-eaters. To what extent the floor of the 

 ocean is peopled stiU remains a difficult question. 

 As already noted. Dr. Hjort beUeves that genuine 

 abyssal fish, in the sense of groimd forms, are few, 

 most of the so-caUed abyssal forms being bathypelagic 

 and having been obtained in the process of hauling 

 in the dredge. But there can be no doubt that many 



