in] THE FACTORS OF DISTRIBUTION 55 



the latter regions. So also with the invertebrates 

 generally : we find the greatest density in the colder 

 seas but there are more bizarre forms and richer 

 colouring in the warmer waters within the torrid 

 zone. Molluscs are much more abundant in the 

 temperate seas, and generally the shells of the cold- 

 water molluscs are less massive in proportion to the 

 size of the animal than in the warm-water species ; 

 thus the largest marine shellfish the great Tridacnas 

 and the mother-of-pearl shells are inhabitants of 

 tropical seas. The pelagic molluscs of the warm waters 

 have generally greatly reduced shells, while those of 

 the northern sub-arctic seas the winged pteropods 

 on which the whalebone whales feed, have a more 

 massive shell. We find also that those animals which 

 secrete lime do so to a greater extent in the tropical 

 seas : thus the reef-building corals are practically 

 restricted to a belt of sea extending to about 20 

 north and south from the equator, while even the 

 solitary corals are not very abundant in the colder 

 seas. The micro-crustacea are probably about equally 

 varied in the cold and warm seas but they are vastly 

 more abundant in the temperate and sub-arctic seas. 

 Marine plants are much more plentiful in the tem- 

 perate and polar seas : the larger rooted Algae are 

 represented by more species and are denser in the 

 temperate seas than anywhere else in the ocean. 

 The pelagic microscopic diatoms, though found 



