120 



LOWER COASTAL ANIMALS 



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Holes and tracks of crabs (Ocypode) on the edge of the dunes. India. 



A Dane, accustomed to no tide in Baltic waters, is invariably impressed 

 by the tremendous part which tides play in the lives of shore animals 

 throughout most of the world. It is the absence of tide which is excep- 

 tional, a difference between high and low tide of anything between half 

 a metre and three metres being normal. Great variations occur in the 

 conditions afforded for animal life in this zone; the drying of the shore 

 is a major factor, and of course the higher up animals live in the tidal 

 zone the shorter the period in which they remain covered, or at least wet. 

 Periwinkles can venture fairly high because they are able to feed above 

 the water level, grazing at night and in humid weather on the minute 

 algae and other vegetable matter which they find on rocks. But there are 

 few periwinkles below the high-water mark, their place being taken by 

 species which cannot tolerate so much dessication and which follow the 

 incoming and outgoing tide, feeding on the rocks at the water line. This 

 applies to the limpet (Patella) and another gastropod with a conical shell 

 (Siphonaria), to the broad, short-spiral genus Nerita so common in 

 all warm climates, and to the coat-of-mail shells or chitons, which resemble 

 limpets but have an eight-plated shell. We found these on every rocky coast 

 observed, always under the same conditions. Every time an individual 

 goes out on the receding tide it will stop at its own particular place, to 



