THE PELAGIAL 



235 



water molecules. These facts explain numerous features of the dis- 

 tribution of pelagic animals. 



The radiolarians of the family Challengeridae (Fig. 58) are as- 

 sorted vertically according to size, the smaller ones in the warmer 

 surface waters, the larger ones below. Thus the average sizes in milli- 

 meters of the following forms may be grouped according to depth: 13 



50-400 m. 



Challeugeria xiphodon 0.11 mm. 

 " strirei 0.11 " 



" harsioni 0.16 " 



400-1500 m. 



Ch. bethelli 0.215 mm. 

 " slogetti 0.25 

 " iizardi 0.28 " 



1500-5000 m. 

 Pharyngella gaslrula 0.33 mm. 



Ch. thomsoni 

 " naresi 



0.35 

 0.58 



The same is true of other radiolarians, for example, Aulacantha 

 scolymantha. This seems like a mechanical sifting, a sinking of the 



Fig. 59. — The copepod Calanus finmarchicus, X 26. The antenna used as 

 an oar at the left, the maxilla (greatly enlarged), with its bristles, at the right. 

 After St. Brady. 



animals to the depths at which they can maintain themselves. Numer- 

 ous Metazoa exhibit the same phenomenon; for example, the arrow- 

 worm, Sagitta hexaptera, which inhabits the lighted pelagial, is larger 

 and more mature, the deeper the source of specimens. 14 The little fish 

 Cyclothone microdon increases in size with depth, specimens from 

 500-m. depth averaging 30 mm., from 1500-m. 60 mm., and specimens 



