260 ANIMALS OF THE SEA FLOOR 



necessarily made from a distance, can appreciate. 

 The reason for the preference for one type of bottom 

 rather than another usually resolves itself into a 

 question of support or of shelter, either for the species 

 itself or for its prey. Of course, there are very many 

 species that may be found indifferently on all sorts of 

 bottom and under all kinds of conditions. 



The connection between the nature of the bottom 

 and its inhabitants may be illustrated by an example. 

 In some spots on the west side of the Irish Sea, when 

 a haul of a trawl is made, at least 90 per cent, of the 

 catch consists of the Alcyonarian or " false coral," 

 known as " Dead Men's Fingers " (Alcyonium digi- 

 tatum), while on the surrounding grounds only a few 

 specimens are to be met with. The reason of this is 

 clear. These spots where Alcyonium flourishes are 

 beds of dead shells, mostly scallops and spiny cockles, 

 which afford a foothold for the swimming larva of the 

 Alcyonarian when seeking a place on which to settle 

 down. Incidentally, the local range of at least three 

 other species is marked out at the same time ; two 

 Nudibranchs which feed on Alcyonium are always to 

 be found in the neighbourhood of these shell beds, 

 and also a small Isopod Crustacean which lives on the 

 same species. 



In the case of organisms which are free to move about, 

 the reasons for one kind of ground being more suitable 

 than another are not so evident. The Norway lobster, 

 or Dublin Bay prawn (Nephrops norvegicus), which 

 abounds on the mud covering the bottom of a great 

 part of the Irish Sea, rarely ventures on to the adjacent 

 sandy grounds ; while the masked crab (Corystes), 

 which frequents a clean sandy bottom, is never found 

 on^the mud. The circular crab (Atelecyclus) is not 

 found on either sand or mud, but should be looked for 



