CH. X] THE CONDITIONS OF LIFE IN THE SEA 241 



I will notice very briefly some of the work that has been 

 carried out during recent years, with relation to influence of 

 changes in temperature, salinity, and light, on the metabolism and 

 habits of some marine organisms. 



Temperature. It is well known that the distribution and 

 migrations of marine animals are partially dependent on the 

 temperature of the sea. The general facts of the geographical 

 distribution of marine species shew this so well that I need only 

 notice the case of the corals. Massive coral formations are only 

 found in those seas where the temperature is not less than 21° C. 

 and where the annual range is not greater than 7^C. The 

 secretion of lime by marine animals is greatly influenced by the 

 temperature of the sea, so that the shells of mollusca are more 

 massive in tropical than in temperate seas. Note the cases, for 

 instance, of the tropical Tridacnas and the mother-of-pearl oysters. 

 Calcareous skeletons are also more attenuated in the case of 

 animals living at the bottom of tropical seas than in the case of 

 those which live at the surface. It is rather difiicult to separate 

 the effects of temperature and salinity but there is no doubt that 

 the former is a factor in itself. Thus the changes of salinity in 

 the Irish Sea are insignificant when compared with those in the 

 North Sea and Baltic, but we nevertheless find that there are 

 seasonal migrations of fishes which appear to be related to seasonal 

 changes in temperature. Thus bass {Lahrax lupus) regularly 

 invade the waters of Morecambe Bay in June and July when the 

 temperature of the sea is approaching the maximum. Soles 

 appear to segregate about the same time, and in the autumn, 

 when the temperature is beginning to fall again, large plaice 

 approach the shore from the deeper waters. Cod come into the 

 Irish Sea when the temperature is falling to the minimum at 

 the end of the year ; and whiting frequent the deeper waters in 

 the northern parts of the same sea just after the annual minimum 

 of temperature has been attained, disappearing soon afterwards. 

 D'Arcy Thompson has shewn that there is a certain correspond- 

 ence in the volume of the catches made by the Aberdeen 

 trawlers, and the temperature of the sea on the fishing grounds 



J. F. 16 



