152 PRIMITIVE FORMS OF LIFE 



theory of preadaptations, that naturally the animals that 

 descended into the darkness of the ocean depths were those 

 that could illuminate it. As, however, the coastal species which 

 may be considered as ancestral are not luminous, we must 

 admit that the illuminating apparatus only developed after 

 their descent, and not dismiss too rashly the idea that the 

 absence of solar light favoured their appearance. Whatever 

 the reason may have been, this faculty of developing luminosity 

 is possessed only by a certain number of the types constituting 

 the deep-sea fauna. 



The deep-sea forms are not directly related. We find 

 numerous vitreous Hexactinellid Sponges, and very few members 

 of the other groups ; Alcyoniaran Ccelenterates of the coral 

 type or the solitary Madreporaria (Flabellum) ; few Bryozoa, 

 but quite frequently doubtful forms assigned to them like 

 Rhdbdopleura and Halilophus dodecalophus . The Crustaceans also 

 abound, and orders that are generally of small size are some- 

 times represented here by gigantic forms, such as Bathynomus 

 giganteus, a large Isopod two decimetres long, or 

 Gnathophausia gigas and Goliath. Annelid Worms are seldom 

 encountered. Molluscs are rare and small, and it is their absence 

 that has led to the peculiar habits of the Hermit-crab, which 

 occurs fairly often. The Crustaceans of this group have large 

 soft abdomens, which they enclose, if they live along the coasts, 

 in hollow shells easy enough to find. As they grow they change 

 their shell in order always to have a house appropriate to their 

 size in which they can be completely sheltered. Quite often a 

 beautiful vivid red Sea-anemone, belonging to the genus Adamsia, 

 instals itself on this shell, and a kind of symbiosis is established 

 between the Crustacean and Ccelenterate. Certain hermit- 

 crabs can lodge in a fragment of bamboo. 1 Some even make 

 out of earth their own mobile habitation, like a kind of 

 caravan. 2 At great depths the Gasteropod shells are rare, and 

 small. The hermit-crabs can get into these shells perfectly 

 well when they are young, but as they grow larger they make 

 no effort to replace them ; then they keep them, merely from 

 habit, in order to satisfy their instinct, although the shells have 

 become useless, and we sometimes find splendid specimens of the 

 hermit-crabs 3 with abdomens the size of a large human thumb, 



1 Xylopagunis. 



2 Pylocheles. 



3 Catapagurus. 



