GEOGRAPHIC DIVISIONS OF THE PELAGIAL 273 



polar seas. The lack of seals in the tropics may have been originally 

 determined by this same phenomenon. 



When the pelagic communities of the tropical seas are examined, 

 it is found that about 20 species of Foraminifera occur there, while only 

 one or two dwarf species are found in polar waters. The Siphonophora, 

 with few exceptions, are confined to the tropics. The Geryonidae among 

 the Hydromedusae and the Charybdeidae among the Scyphomedusae 

 are confined to the warm seas. Of the annelids, the Alciopidae (with 

 the exception of Vanadis antarctica) are warm-water animals. Of the 

 crustaceans, the species of the copepod genus Cojrilia, which accumu- 

 late toward the equator, most of the Euphausiacea, most of the Ser- 

 gestes, as well as the shrimp Lucijer, are present only in the warm 

 water. Of the mollusks, all the heteropods, most of the pteropods, and 

 the pelagic snails Ianthina and Phylliroe, are limited to warm water. 

 Among the tunicates the Pyrosomatidae belong to the warm waters 

 entirely, as do the salpas with few exceptions. Thirty species of Ap- 

 pendicularia in the Atlantic, representing seven to eight genera, occur 

 in the warm zone, as against three species (of two genera) in the cold, 

 and the latter are specifically different. The tropical plankton includes 

 numerous free-swimming larvae of forms of echinoderms, mollusks, 

 annelids, and others, which are entirely absent in cold water. Finally, 

 the flying fishes {Exocoetus and its relatives) are strikingly charac- 

 teristic of warm seas. It is characteristic of the shifting boundaries of 

 the warm-water belt that the northern limit for flying fish extends 

 about 5-8° latitude farther north in the northern summer than in the 

 northern winter. The absence of flying fishes in cold seas is not surpris- 

 ing; the swift movement of the moist animals through the cold air 

 would cause too rapid cooling. 



It is worth noting that in spite of the complete separation of the 

 warm-water belt into two subdivisions, the Atlantic and Indo-Pacific, 

 separated by the continents of Africa and America, the pelagic animal 

 life limited to the warm water shows a great agreement in both areas. 

 Thus, for the copepod species occurring in both, part are identical* and 

 part of them are nearly related species which vicariate in the two re- 

 gions;! a similar situation exists among the schizopod and descapod 

 crustaceans; 4 most of the salpas, and certainly the most abundant 

 species, are present in both oceans. The significance of the connection 

 of both oceans in the region of Central America during Tertiary times 

 in this connection is evident. 



*Corycaem lautus, C. specious; Calanus robustus, C. vulgaris; Heterochaeta 

 papilligera. 



t Like Corycaeus clausii and C. vitreus. 3 



