STUDIES IN MARINE ECOLOGY. 243 



character of bottom, depth, temperature, salinity, purity of 

 water and currents as the important factors in determining 

 distribution. The first of these factors, character of bottom, is 

 characteristically different in the associations of eroding and 

 depositing shores. The former is characterized by the presence 

 of firm places for attachment and difficult burrowing conditions ; 

 the latter, by the converse of these conditions. The substratum 

 also serves largely in distinguishing between the different asso- 

 ciations of each series. 



The influence of depth as a factor in animal distribution in the 

 region covered by these studies is not due to depth as such but 

 to depth as assuring a constant supply of water. This is shown 

 in the rockweed association where animals normally found below 

 low tide level on the wharf pilings may live well above it when 

 protected from drying and from high temperatures by the mat 

 of rockweeds. 



Temperature serves as a limiting factor for these associations, 

 during the season of the year studied, in the tide pools and more 

 particularly on the flats. There in the Scoloplos acutus associa- 

 tion above the dense eel grass mats at low tide the high tempera- 

 ture (32 C.) that may be reached must serve to kill off the 

 more sensitive sessile animals as it drives the motile ones below 

 the surface layer to the cooler water in the shade of the eel 

 grass. The temperature to which animals may be exposed at 

 low tide increases as one leaves open water conditions in both 

 series. The effect of this high summer temperature of the flats 

 as a factor limiting the geographic distribution in this region 

 will be discussed in Study IV. 1 of this series. 



In the associations studied, the salinity regularly increased as 

 one approached open water conditions. The low salinity on the 

 back part of the flats particularly in the Melampus, Mya and 

 Thyone associations must serve as a limiting factor. These are 

 subjected to such extreme ranges of salinity following heavy 

 rains, particularly if these rains come at low tide and where 

 there is some considerable surface drainage, that sensitive 

 animals or animals in a sensitive stage in their life history must 

 needs be killed or driven off. 



In the locations studied, there was no contamination from 



1 In press in Ecology. 



