278 MARINE ANIMALS 



demonstrated in the abyss of warmer seas, as for example, Homoenema, 

 Atolla, Periphylla. 22 



The bipolar ctenophores, Pleurobrachia pileus and Beroe cucumis, 

 are also found in the intermediate zones, sometimes at the surface 

 and sometimes in the depths. Of the annelids, 23 the polar seas show 21 

 species in common, of which a few have since been found close to the 

 equator. Of the gephyrean worms, 5 identical species occur at both 

 poles, and 3 of these have already been taken in the abyssal waters of 

 the low latitudes; still other species which occur in the Arctic and in 

 the abyssal waters to the south are not yet known in the Antarctic, 

 perhaps not having arrived there yet in their dispersal. 24 The bipolar 

 arrowworm, Krohnia hamata, lives in the deeper pelagic strata of 

 warm seas. Of 6 species of copepods 25 common to both poles, 5 also 

 occur in intermediate regions; thus, for example, the commonest of the 

 northern copepods, C alarms finmarchicus, also appears in the southern 

 part of the Atlantic and was taken in the Sargasso Sea at depths of 

 650-1500 m. 26 Among the amphipods, 27 a few species are identical in 

 the Arctic and Antarctic; a few of these, for example, Orchomenopsis 

 chilensis, have been demonstrated in the tropics in deep water. The 

 pteropods Limacina helicina and Clione limacina aside, only those 

 species of mollusks are common to both poles that are also present in 

 warm seas.* The arctic and antarctic shark, Squalus blainvillei, 29 has 

 a half abyssal habitat. The bony fishes, Lycodes, Gymnelis, and 

 Melanostigma, 30 which are common to both poles, are connected in 

 their distribution by means of the deep sea. 



But in other cases, such a direct connection may be lacking, and 

 the identity of the polar forms is to be referred to the convergent 

 evolution of species, which live in the transition areas in warm weather. 

 The thick-shelled Globigerina pachyderma, which is found in both 

 polar seas, is a local subspecies of G. dutertrei, a species distributed in 

 the transition region, which develops into pachyderma under the in- 

 fluence of low temperature. 31 The appendicularian Frittilaria borealis 

 has a world-wide distribution, but is represented in the polar seas by the 

 identical variety, F.b. borealis. 32 Of the cladocera, there is evidence 

 that because the forms in the tropics are so small they were described 

 as separate species and their relationships with their polar repre- 

 sentatives were not recognized. 27 The same may also apply to the 

 isopods, of which a large number of genera are bipolar. 



But it is important to note that there are many groups in which 



* The following may be mentioned: the mussels, Saxicava arctica, Lasca rubra, 

 Puncturella noachina; the snail, Natica groenlandica ; and the elephant tusk, 

 Dentalium entails!* 



