THE SEA BOTTOM 73 



It might appear at first sight as though this knowledge 

 was of quite secondary importance and not worth the very 

 great labour its collection undoubtedly entails. But this is 

 far from being the case. We have already referred to the 

 unity of life, and here is an example ; the bottom animals, 

 especially the shellfish and worms, form the most important 

 part of the food of the bottom-living fish, such as the different 

 flat-fish, the skates and the haddock, and it has been shown 

 by the Danish investigators that the success or failure of the 

 fisheries in any year is largely dependent upon the numbers 

 of other members of the bottom fauna, especially shellfish, 

 in that year. On the other hand the prevalence of starfish 

 has just the opposite effect for these animals — from the 

 fisherman's point of view — are pests, pure and simple, they 

 are of no use to man while they destroy countless numbers 

 of shellfish. 



Adaptations 



Different animals are adapted for life under different 

 conditions, and the bottom-living aniamls possess many 

 structures and peculiarities which fit them for life on the 

 sea bottom. We have already spoken of the manner in 

 which burrowing animals maintain connection with the 

 surface, while in Chapter IX we shall see something of their 

 devices for obtaining food. There is also the vital matter 

 of reproduction. Very many of them produce eggs which 

 hatch out into animals totally unlike their parents. Star- 

 fish and their relatives provide the most striking examples of 

 this, but worms and crustaceans afford instances almost as 

 striking (compare the adult crab in Plate 14 with the young 

 of the same species shown in Plate 43) . These "larva?" do not 

 stay at the bottom but rise towards the surface, where they 

 are for a time part of the drifting life, described in Chapter V, 

 and only settle down to the humdrum life of their parents 



