GENERAL CONSIDERATIONS. 387 



phores as well. In the case of the latter group there has been a small northerly 

 spread, and an even smaller expansion downward. Wliether there has been 

 any extension into the frigid zone of the southern hemisphere remains to be 

 seen, though it is hardly conceivable that there should not. On this point, 

 as on many others, we can expect much valuable information from the collec- 

 tions made by the "Valdivia." 



The Siphonophores have been less successful in occupying cold waters than 

 either Medusae or Ctenophores. The trachomedusan, narcomedusan, and 

 scyphomedusan fauna of the Arctic and Antarctic waters, of which Maas ( : 06) 

 has given a general account, are qualitatively rich, and quantitatively even more 

 so. And when we turn to Ctenophores, we find that of eighty species recognized 

 by Moser (: 09) as valid, nine are known from the Arctic, or from the Antarctic. 

 But among the rather larger hst of Siphonophores (about 95), only three species 

 have any claim to be regarded as normally Arctic forms. Specimens of other 

 species, it is true, are occasionally taken far north, as for example Physophora 

 and Diphyopsis dispar. But these records are nothing more than sporadic 

 instances of the effects of currents. The animals have not succeeded in estab- 

 lishing themselves there. The difference between Siphonophores and Cteno- 

 phores in this respect is further emphasized by the fact that it is doubtful whether 

 there is a single truly eurythermal Siphonophore, while there is at least one 

 Ctenophore, Pleurobrachia pileus, the distribution of which is wholly independent 

 of temperature. And the same is probably true of a second, Beroe cucumis. 

 For the former, the known temperature range is from just above freezing to 

 28° C (82.4° F.) ; for the latter, from the same low limit to 23° C. (73.4° F.) . The 

 extreme temperature range known for the most nearly eurythermal Siphonophore 

 is from about 45° F. to about 80° F. ; considerably less than either of these Cteno- 

 phores, while its normal range, from about 55° to about 80°, is still narrower. 



Another significant fact is that the Siphonophore component of the Arctic 

 plankton is not only qualitatively, but quantitatively, much poorer than either 

 the Medusae, the Ctenophore, the Pteropod or the Chaetognath component. 

 Thus Diphyes arctica occurs far less regularly in Arctic currents than do the 

 Pteropod Clione limacina, the Chaetognath Krohnia harnata, or the Tracho- 

 medusa Aglantha digitale, though it has been captured from a wide range of 

 localities. Nor has it ever been found so abundant anywhere as Clione, Aglantha, 

 Pleurobrachia, or Mertensia, which often gather in enormous swarms. The 

 same thing is true, also, of Stephanomia cam, though it is large and conspicuous. 

 And this species is even more irregular in its appearance. 



