MAEINE BIOLOGICAL STATION AT POET ERIN. 291 



animals over the sea-bottom within one zoological area. 

 It is certainly more important than mere depth; a muddy 

 bottom will support a similar fauna at 10 fathoms in one 

 place and at 50 fathoms in another. Probably the most 

 important influence in the environment of a lower animal 

 is its food, and once beyond the narrow sub-littoral zone 

 in which algae flourish — and to which, of course, certain 

 phytivorous animals must be restricted — it is probably 

 chiefly the nature of the bottom which determines the 

 food.* Many animals feed upon the deposit, others 

 browse upon the polyzoa and zoophytes which can only 

 attach themselves and grow where there are sufficiently 

 large objects, such as shell valves, from which they can 

 get the necessary stability; while others, again, feed upon 

 their neighbours, which subsist on the deposit or are 

 attracted by the zoophytes, &c. ; for example, soles are 

 frequently caught upon ground (known to fishermen as 

 " sole ground ") where Flustra foliacea lives in abundance, 

 and the probable connection is that the fish are dependent 

 upon the numerous amphipoda and other small aninaals 

 which frequent the tufts of Flustra. The same locality 

 may vary so much from time to time in the temperature, 

 the salinity, and the transparency of the water, that it is 

 probable that none of these factors — so long as the varia- 

 tions do not exceed certain limits — have so much influence 

 upon the fauna as the nature of the deposit has. It is 

 therefore quite to be expected that the fauna should vary 

 from place to place with the nature of the bottom, and 

 that is what we have observed frequently in our work- 

 round the Isle of Man. In practically the same water, 



* The only food supply quite independent of the bottom is dead phinkton, 

 from the water above, which may reach the bottom uneaten ; and possibly a 

 small amount of decayed vegetation and other organic matter brought down 

 by rivers from the land, and some of which may reacli the sea-bottom. 



