SPECIAL PROBLEMS OF SHORE LIFE 77 



of the day the currents of the fresh-water sponge Spongilla 

 cease, and the oscular collars are somewhat contracted. 

 This is a response to unfavourable conditions analogous 

 to the retraction of an anemone at low tide. 



Mayer {loc. cit.) observes that it is easy to see why certain 

 forms are confined to the tropics as they lose all power 

 of movement at from 10° to 12° C. ; indeed, he thinks it 

 probable that no single factor is a more effective barrier 

 to the extensive geographical range of marine animals than 

 is that of temperature. The temperature factor is, of course, 

 not always decisive, since, as Mayer points out, the tropical 

 Limulus Polyphemus from Florida survives being frozen into 

 the ice and from near its northern limit (off Massachusetts) 

 continues to move until heated to at least 40° C. Yet it is 

 not world-wide, only ranging from Maine to Yucatan. 

 Interesting also is the fact, noted by the same writer, that 

 when a boreal animal becomes acclimatised to the tropics 

 its upper death temperature is raised, and, conversely, it 

 becomes unable to withstand a degree of cold in which its 

 northern relatives may thrive. Its optimum temperature 

 is, however, raised even more conspicuously than its death 

 points, and thus its factor of safety against abnormally 

 high temperatures is reduced. Analogous observations have 

 been made at Naples, where Vernon (1899) found that 

 the mean death temperature of various invertebrates ranged 

 from 34° to 42*3° C, and that it was higher for the same 

 species by o"6° to 1*3° in July- August than in March- April, 

 this greater tolerance being associated with a rise in surface 

 temperature of about 10° C. 



Marine animals of the temperate and arctic zones seem 

 relatively more tolerant of excessive cold than do tropical forms 

 of abnormally high temperatures. Thus, Cyanea arctica, for 

 instance, continues to pulsate even when half its bell is 

 frozen into the ice, and after being embedded solidly for 

 several hours it revives at once, apparently uninjured, when 

 the ice has melted. 



The above facts are mentioned not for their bearing on 

 the distribution of marine animals, a subject with which 



