556 DEPTHS OF THE OCEAN 



temperature. Experiments have taught us that the eggs of 

 CuctLmaria, which float near the surface, are so much affected 

 by the surface-temperature of the coast-water in summer, that 

 they are destroyed before a single larva is hatched, and it 

 follows that the existence of this form in the littoral zone of the 

 boreal region depends upon its period of reproduction being in 

 the coldest months of the year ; this is probably true also of 

 Echinaster. Again, in the case of another arctic-boreal species, 

 Hippolyte gamzardi, which along the west coast of Norway 

 lives only in the littoral zone, the eggs develop during the cold 

 months of the year, and the young are hatched in April. On the 

 other hand, the lobster and the oyster, which are typical boreal 

 forms inhabiting the littoral zone, have their period of repro- 

 duction in the months between June and August.^ It must be 

 admitted, however, that too few researches have been made upon 

 which to base any general conclusions, and that the conditions 

 in arctic tracts are quite unknown." 



Little is known as yet regarding the power of withstanding 

 variations of temperature in different species, though most of 

 the littoral animals, which are eurythermal and exposed to 

 extreme variations, are astonishingly hardy. The Swedish 

 zoologist Aurivillius has found, from observations made on 

 the coast of Bohuslan in Sweden, that the barnacle (Balanus 

 balanoides), the periwinkle i^Littorina littorea), the sandgaper 

 {Mya), the cockle {^Cardiuvi), and the lugworm [Arenicold) are 

 able to endure for a considerable period a temperature below 

 freezing point, and that the barnacle after being quite a 

 long time in the ice had actually got vigorous young.^ Other 

 littoral forms can protect themselves by descending into deeper 

 water or by burrowing downwards into the mud. Still we 

 cannot expect every species to be equally hardy, and wholesale 

 destruction sometimes takes place under specially unfavourable 

 circumstances, as, for instance, when the ice lasts too long or 

 when the bottom freezes to too great a depth. That many of 

 our littoral animals are able to live in boreo-arctic areas at a 



^ The German naturalists Samter and Weltner have published an interesting account 

 of several arctic survival forms in North German lakes, illustrating their mode of life and 

 reproduction. One crustacean, Mysis relicta, lives during the summer in the depths of cold 

 lakes, and migrates landwards during autumn and winter, reproduction chiefly taking place at 

 a temperature of 3° C. With another crustacean survival-form, Pontoporeia affiftis, also, repro- 

 duction takes place m winter at temperatures varying between 0° and 7° C. 



- It will be interesting to find out whether the boreal forms which penetrate into boreo-arctic 

 areas with high temperatures for a short portion of the year have a short period of reproduction 

 there, seeing that farther south their reproduction is known to extend over several months. 



^ Aurivillius, " Littoralfaunans furhuUande vid tiden fiir hafvets islaggning," Cfvers. Kgl. 

 Vet. Akad. Forhandl., 15^95. 



