INVERTEBRATE BOTTOM FAUNA 555 



instance, that a high temperature is required for the develop- 

 ment of the oyster larvae, and that along the Scandinavian coast 

 it is only in the so-called pools that reproduction on any large scale 

 takes place. Most probably the same is the case with many 

 other inhabitants of the pools. The eggs and larvae of the lobster 

 are only developed during the warmest part of the year, though 

 the female often carries spawn in winter, and it has been found 

 by experiment that a fall of a few degrees in temperature is 

 sufficient to retard the development of the larvae several weeks. 

 We can understand, therefore, why these forms do not live in 

 arctic or boreo-arctic areas. Even though the fertile eggs or 

 larvae of boreal forms do not demand a higher temperature 

 for their development, additional warmth may nevertheless be 

 absolutely essential for the production and development of the 

 ova within the mother's body. This, again, limits the dis- 

 tribution of many forms. The converse naturally holds good, 

 and the development and other physiological processes of forms 

 living exclusively in arctic waters can only take place at a low 

 temperature. 



We have already seen that many species are common to 

 both boreal and purely arctic areas, and we must ascribe their 

 widespread distribution to their power of adapting themselves 

 to very different temperatures. Most likely we are dealing 

 here with physiologically distinct species, even though the 

 differences do not appear in corresponding morphological altera- 

 tions in bodily structure. Not that differences of this latter 

 kind are by any means excluded, as I have previously shown 

 how a species may vary morphologically in certain directions, 

 according as it occurs in arctic or boreal tracts. Future 

 researches regarding the time when reproduction begins in 

 these widespread forms in the respective areas will possibly 

 show that the temperature at which development takes place 

 varies a good deal less than the temperature prevailing in the 

 different areas seems to indicate. For forms which live in 

 boreal deep water, where the temperature is comparatively low all 

 the year round, the difference is in any case not particularly 

 great, and if it should prove that the widespread shallow- 

 water forms develop during the winter in boreal areas, the 

 difference there again is relatively small. Now we find that two 

 of our typical littoral animals, the sea-slug Cuannaria frondosa 

 and the starfish EcJiinaster {C^'ibrelld) sanguinolentus, both of 

 which inhabit arctic tracts, deposit their eggs in boreal waters 

 early in March when the upper water-layers have a low 



