ANIMAL LIFE 165 



Again, the secretion of carbonate of lime 

 by organisms is very much retarded in cold 

 water as compared with warm water. In cold 

 water it is believed that calcite is laid 

 down, while in warm water aragonite is 

 deposited. As an illustration it may be 

 mentioned that in the tropical waters there 

 are about 35 species of shelled pteropods and 

 32 species of heteropods. These gradually dis- 

 appear as we proceed from the equator towards 

 either pole, till in truly polar waters only one 

 small shelled species of Limacina is to be found 

 in the Arctic and another allied species in the 

 Antarctic. Meisenheimer states that the 

 pteropod Limacina helicina occurs both in 

 the Arctic and Antarctic, while Limacina 

 retroversa occurs in the northern and southern 

 temperate regions, though absent from the 

 warmer tropical belt. Some authors, on the 

 other hand, do not regard these northern and 

 southern forms as identical, but rather as 

 distinct varieties. 



Quite similar is the distribution of the 

 pelagic foraminifera. Towards the equator 

 more than twenty species may be found in the 

 tow-nets, but they gradually disappear as we 

 proceed into colder water towards the poles ; 

 only one species of Globigerina (G. pachyderms) 

 is found living in the Arctic surface waters 

 and another species (G. dutertrei) in the Ant- 



