ON SHALLOW WATER FAUNAS. 33 



of Professors Moseley and Semper I am under very special 

 obligation. 



The shallow water fauna is found in the greatest profusion, 

 and in its most characteristic form, between tide marks, and 

 at depths extending from low water level down to about fifty 

 fathoms. From fifty to two hundred fathoms it occurs in 

 less typical condition, and below the latter depth passes 

 gradually into the deep sea fauna. 



The most marked features of this shallow water fauna are 

 the great profusion and variety of forms which it presents. 

 Representatives of nearly every one of the great groups of 

 animals occur, and the actual number of species very greatly 

 exceeds that of the other ocean faunas. 



As regards environment, the chief point is its extrerne 

 variability, which stands in the most marked contrast with 

 the almost absolute uniformity of external conditions affecting 

 the deep sea fauna. Thus, animals living between tide marks 

 are exposed daily, and may spend half their lives in and half 

 out of water, while both they and animals living at rather 

 greater depths are subject to changes of enormous violence at 

 times of storms. Again, shallow water animals, especially 

 those that are periodically exposed, are liable to considerable 

 variations of heat and cold, and of light and darkness. 



The varying nature of the sea-bottom, whether rock, sand 

 or mud ; the proximity of great rivers, or other causes that 

 may modify the quality of the water ; and the various changes 

 effected round the coasts by human agency, all help to swell 

 the list of causes that render the environment of shallow 

 water animals singularly inconstant. Another special feature 

 is the abundance and variety of plant life, affording a copious 

 supply of food denied to the inhabitants of deeper waters. 

 The actual bathymetrical distribution of plants is less 

 accurately known than that of animals, but the ordinary 

 seaweeds do not extend below fifty fathoms, while below 



c 



