vii THE ANTARCTIC AND CIRCUMTROPICAL ZONES 2OI 



my sis scypliops, which occurs at both poles, but is not known from 

 the tropics. This is a pelagic species, and we know that the 

 Mysidae often descend to considerable depths. We also know 

 that the Mysidae are' dependent on cold water, only occurring in 

 boreal or temperate waters. We may safely suppose, therefore, 

 that the migration of this species has taken place by their for- 

 saking the surface-waters as the tropics were approached, and 

 passing down into the depths where the temperature is constantly 

 low even in the tropics. 



The dependence of Crustacea upon the temperature of the 

 water is also illustrated by the distribution of the Lithodiuae. 

 The headquarters of this family are in the boreal Pacific, with a 

 few scattered representatives in the boreal Atlantic. The cool 

 currents on the western coasts of America, however, have per- 

 mitted certain forms to migrate as far south as Patagonia, where 

 they still have a littoral habit. In the tropical Indo-Pacific, 

 where a few species occur, they are only found in deep waters. 

 Thus _ at these various latitudes, by following cool currents or 

 migrating into deep water, they are always subjected to similar 

 conditions of temperature. The same kind of thing is observed 

 in Arctic seas, where deep-sea forms are apt to take on secondarily 

 a littoral habit owing to the temperature of the depths and of 

 the shore being the same. 



Despite the impassable barriers of land which now sever the 

 tropical oceans, we can yet speak of a circumtropical zone 

 possessing many species common to its most widely separated 

 parts. Such circumtropical species, occurring on both the Atlantic 

 and Pacific coasts of tropical America, on the West African coast, 

 and in the Indo-Pacific, axe various Grapsidae, Oalappa granidata 

 and its allies, and certain Albunea. The most striking instance 

 of all is that of the Land-crabs. Of Oeypoda, the greater number 

 of species occur in the Indo-Pacific, but representatives are also 

 found on the tropical Eastern and Western American coasts and 

 on the West African coast, and the same is true of Gelasimus. 

 The genus Cardisoma, belonging to a different group of Land- 

 crabs, is also typically circumtropical. 



For this community of the circumtropical species we may 

 certainly advance in explanation the comparatively recent forma- 

 tion of the Isthmus of Panama. Besides the resemblance of the 

 Crustacea on the east and west coasts of the isthmus, we have an 



