58 THE SBAS 



one to exist in mud and the other in shell gravel. It hai 

 been found that the free swimming larvae of certain marina 

 worms, although ready to metamorphose to the adult 

 Btructure and settle on the bottom, will delay this if a suit- 

 able substratum is lacking and continue to swim about and 

 to grow until a suitable bottom is found. In the second 

 place the bottom material may contain important re- 

 serves of lime, silica and manurial salts, although modem 

 investigation shows that actually the products oi 

 the decay of animal and plant life are quickly returned 

 to the water. 



Life on the Sea Bottom 



The animals which inhabit the bottom ot the ocean ar« 

 called the benthos, to distinguish them from the driftmg 

 and actively swimming animals known as plankton and 

 nekton respectively. Extensive investigations by means 

 of dredges and trawls have shown that the benthos 

 varies greatly in different regions both in quantity and 

 quality. Generally speaking animals are most numerous in 

 coastal waters, and become less and less so the further we 

 pass from the land into deeper water, though there is 

 animal life, and often of a unique and interesting kind, 

 even in the abyssal regions with red clay deposits. The 

 greatest number of different species is found in the tropics, 

 especially around continents and in the neighbourhood of 

 coral islands, but the temperate and polar seas make up for 

 the paucity of variety of species by the far greater 

 numbers of individuals of those kinds which occur there. 



Differences in fauna are chiefly the result of dififerences in 

 depth, temperature, salinity and food, and of the interactions 

 of these factors. The influence of the last is universally felt, 

 that of salinity is apt to be local, the result of currents or 

 great influxes of fresh water from rivers, that of temperature 

 is especially eifective in the shallower waters near the coast 



