A. EARLAND ON RADIOLARIA. 279 



cross-bars of the network thinner. The armature of spines is 

 also more varied and developed. Many pelagic Kadiolaria have 

 either an incomplete skeleton or none at all. 



More than half of all the Radiolaria known belong to the 

 abyssal fauna, from depths between 2000 and 4000 fathoms. 

 They live constantly at this depth, floating at a little distance 

 above the sea bottom, and have evidently become specially 

 adapted to the extraordinary conditions of their existence — utter 

 darkness except for such light as may proceed from their own 

 phosphorescence or that of other deep-sea animals, accompanied 

 by a constant temperature but slightly above the freezing point, 

 and tremendous pressure. Probably as a result of these con- 

 ditions of life, they present certain structural features by which 

 they may be distinguished from the pelagic and zonarial forms, 

 whose skeletons also occur in prodigious numbers in deep-sea 

 deposits. The lattice-work of the shells is coarser and thicker 

 and the pores smaller than in pelagic species of the same group, 

 while the armature of spines is less highly developed, and the 

 whole skeleton smaller and more massive. Nearly all the fourth 

 legion, the Phreodaria, belong to the abyssal fauna, which also 

 includes many of the third legion, the ISTassellaria, while, on the 

 ■other hand, the first and second legions are but sparingly 

 represented. 



By the use of tow-nets, lowered to different depths at the same 

 station, it has been proved that the water intermediate between 

 the surface and the bottom is inhabited by zones of Radiolarian 

 life which present characteristic differences at various depths. 

 They may be compared to the zones of vegetation on the sides of 

 a mountain range. From the comparatively few data available 

 Haeckel has provisionally distinguished five zones : — 



(1) The pelagic zone, from the surface to about 25 fathoms. 



(2) The 2)ellucicl zone, from 25 to 150 fathoms, the greatest 

 •depth at which sunlight makes its influence felt. 



(3) The obscure zone, from 150 to 2000 fathoms; from the 

 ■depth at w^hich sunlight vanishes, to that at which calcareous 

 organisms begin to disappear, owing to the solvent action of the 

 carbonic acid in solution. 



(4) The siliceous zone, from 2000 or 2500 to 3000 fathoms, 

 in which only siliceous rhizopods are found, and in which the 

 peculiar influences of the lowest depths have not yet appeared. 



