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But with increasing depth we observe less vegetation, 

 or should algae drift to sea they subside to the bottom, 

 constituting that soft black mud seen in some of our 

 bays, in which worms, molluscs, crustaceans, and other 

 marine animals have their home, and find subsistence, 

 while they in their turn become food for fishes which 

 rout out and consume them. Professor Mobius, at ninety 

 to ninety-five fathoms in the Baltic, where the bottom 

 is formed of plastic clay, found very few worms. In 

 the Mediterranean, at the south-east of Sicily, where the 

 bottom consists of yellowish clay, the British exploring 

 expedition detected no trace of animal life. But in the 

 southern portions of the North Sea, at from twenty to 

 twenty-five fathoms, and with a muddy bottom, such is found 

 to be alive with worms, small Crustacea, snails, molluscs, 

 echinoderms, and fish, and, as a result, with fish which prey 

 upon them. Irrespective of the foregoing algae found in the 

 ocean, there may be microscopic forms so abundant as to 

 render the surface almost turbid. Diatoms live in every 

 sea, their remains forming the principal portion of the 

 finer particles at the bottom, they are swallowed by Pelagic 

 animals, as salpae, pteropods, &c., which after death subside 

 in the waters. Likewise all rivers carry organic matter into 

 the ocean, rendering the bottom rich in food, and a resort 

 for invertebrate animals, and consequently fish which prey 

 upon them. Currents which carry plants and small marine 

 creatures from place to place, must likewise influence, 

 through food, the migration of fish, for the temperature, 

 salinity, and currents of the sea exercise a great influence 

 upon animal and vegetable life. 



Although the growth of fishes much depends upon the 

 suitability and abundance of the food which they are able 

 to obtain, other causes likewise play an important part. In 

 some forms, as the turbot, it would appear as if they aug- 



