PELAGIC ANIMAL LIFE 



587 



fjords along with numerous species of Pandalus, "the deep-water prawns," 



which are now the object of import- 

 ant fisheries. Hymenodora is known 

 / / Vk even from the ice-region, and was 



met with by Scoresby during his 

 arctic voyages.^ 



Though the Mollusca are widely 

 distributed and represented by a 

 vast number of different forms on 

 the ocean-floor, the pelagic forms 

 are comparatively few, but as re- 

 gards abundance of individuals few 

 groups of pelagic animals can com- 

 pare with the winged snails or 

 Pteropoda, which are divided into 

 two groups : Thecosomata (or shelled 

 pteropods) and Gymnosomata (or Pteropoda. 

 naked pteropods). 



The Thecosomata are important 

 on account of the part they play 

 both in the plankton and in the 

 bottom-deposits (see Chapter IV.). 

 They include the family Limacinidae 

 having a spiral shell, of which the 

 well-known Liinadna Jielicina occurs 

 in immense quantities in the Arctic 

 (the seas around Spitsbergen and 

 Greenland), while Liniacina balea, 

 the " Flueaat " of Norwegian 

 fishermen, is a boreal species, and 

 Liniacina retroversa (Fig. 429) is a 

 more southern form occurring also in 

 the Norwegian Sea. The shell is about the size of a pin's head, and can 



Fig. 427. 



Polycheles sculptus pacijicus, Fax. (From Faxon.) 



Fig. 428. 

 Hymenodora glacialis, Buchholz. (From G. O. Sars. ) 



^ In the pelagic life of the ocean the Insecta are represented only by several species of 

 Hemiptera {Halobates and Halobatodes), which are found skimming over the surface in the 

 tropical regions. 



