Lhuntsman] INTERTIDAL ANIMALS 59 



species as the whelk, sea-urchin and star-fish. The two latter forms 

 show in their early stages a greater adaptability to such conditions 

 than they do later in their life-history. This is fairly general among 

 animals which are pelagice in their early stages, the pelagic habit 

 usually involving a life near the surface. 



These three species of animals are predatory in their habits, 

 and for that reason their distribution will affect in an adverse manner 

 the distribution of the animals on which they feed. The starfish 

 and whelk are enemies of the mussel and it may well be that their 

 presence in abundance near and below low-tide mark at St-Andrews 

 is the determining factor for the lower limit of the distribution of the 

 mussel in that region. The zone of common distribution is quite 

 narrow— about three feet— and does not contain any of these species 

 in abundance. It is to be considered the area of struggle— by the 

 starfish and whelk against the effects of heat and exposure, and by 

 the mussel to obtain a foothold in spite of depredations by the whelk 

 and starfish. The lower limit of the distribution of the mussel is not 

 determined by insufficient exposure to air, seeing that in many places 

 it is constantly submerged, for example on floats, mooring poles, and 

 floating breakwaters at St. Andrews. The upper limit of its distri- 

 bution is undoubtedly the highest level at which it is covered with 

 water a proportion of the time great enough to protect it and permit 

 of its growth. The data indicate that under the warmer conditions 

 at Cheticamp it is necessary for it to be covered a greater proportion 

 of the time than under the cold conditions at St. Andrews. Experi- 

 ment will be necessary to determine the influence of each of the varied 

 factors that here come into play. 



The lower limit in the distribution of the barnacle at St. Andrews 

 was found to be in part at least very clearly dependent upon the 

 upper limit in the distribution of the sea-urchin. On the north side 

 of the wharf where the sea-urchins moved up from the ledge each 

 flood tide, the extent of their excursions was plainly to be seen in the 

 form of a more or less semicircular area, of which the ledge formed the 

 centre, and which was entirely devoid of barnacles, thus forming a 

 marked contrast with the white, barnacle-covered remainder of the 

 side of the wharf. The barnacle is extraordinarily resistant to exposure 

 and is able to live near the upper limit of the intertidal zone. Its 

 upper limit of distribution appears to be higher at Cheticamp than at 

 St. Andrews, a difference which deserves investigation. 



We desire to call attention to the opportunities that exist on 

 our coast of studying the factors that determine the abundance and 

 distribution of many of the common marine species. Those that we 

 have considered are all of economic importance, the periwmkle. 



