ANTARCTIC ZOOGEOGRAPHY 369 



mentioned tunicates, hydroids, holothurians, annelids, amphipods", 

 and schizopods. Among the last, the Euphausiids, or opossum 

 shrimps, which have a non-calcareous carapace, exist in inconceivable 

 myriads and furnish food for vast numbers of vertebrates. The 

 secretion of calcium carbonate is determined mainly by temperature. 

 We see an effect of the cold environment in the feeble development 

 of large moUuscan shells or limy skeletons either in Antarctic waters 

 or in the deep sea. Even the shelled pteropods of warm oceans tend 

 to be replaced to southward by naked forms. 



The invertebrates do not divide geographically so as to indicate 

 a definitely Antarctic group, but many specializations similar to the 

 foregoing illustrate adaptations to Antarctic life conditions. The 

 richness -of life in the southern oceans is indicated by figures which 

 Murray^^ records of collections made in the vicinity of Kerguelen 

 Island during the Challenger expedition. The number of species of 

 metazoa obtained at eight stations in depths exceeding 126 fathoms 

 was 272. More remarkable is the fact that not one species among these 

 proved common to all eight stations, and not more than 40 of them 

 were common to any two stations. From such data Murray inferred 

 that further dredgings in the deep water toward the Antarctic would 

 yield a large number of new species, a prophecy which has been 

 abundantly fulfilled. He called attention, furthermore, to the fact 

 that the vicinity of the mud line, at a depth of about a hundred 

 fathoms, is especially rich and that the change in the nature of 

 bottom deposits which occurs beneath the border of the pack ice is 

 responsible also for a transformation in the benthic fauna. Another 

 general condition seems to be substantiated by the southern marine 

 invertebrates, as well as by fishes and higher organisms, namely, that 

 while short distances may reveal well-marked faunal changes in the 

 sub-Antarctic zone, the greater proportion of the truly Antarctic fauna 

 is characterized by circumpolarity. Pelseneer,^^ for instance, notes 

 many circumpolar species among truly Antarctic mollusks, while 

 among sub-Antarctic forms he finds specific change within small 

 distances to be the rule. 



The Austral Fishes 



According to Regan, ^^ all of the fishes native to waters beyond the 

 south temperate regions may be assigned (Fig. i) to either the sub- 

 Antarctic zone, with two districts which he designates by the names 

 Magellan and Antipodes, or to the Antarctic zone, which is like- 

 wise divided into two districts, a Glacial and a peri-Glacial. The 

 annual surface isotherms of 6° C. and 12° C. mark, respectively, the 



^^ Work cited in footnote 9, above, 

 '■i Worli cited in footnote 15, above. 

 35 Worlc cited in footnote 2, above. 



