132 



GEOLOGICAL BIOLOGY. 



Zones of Environment in which Gastropods are Distributed. — 



If we arrange their environmental conditions in tabular order 

 we have the following series, viz. : 



1st. Abyss of the ocean, or an abysmal zone, extending 

 from 500 metres, or 250 fathoms, to the lowest known depths 

 of the ocean. 



^. / . / 



Fig. 35. — Schematic Mollusk. (After I.ankester.) a, tentacle ; i5, head ; c, margin of manfle ; </, 

 margin of shell ; <?, edge of body ; _/, edge of shell depression ; ^, shell ; f^c. cerebral gan- 

 glion; spe, pedal ganglion; gpl, i leural ganglion ; //, osphradium ; /, ctenidium ; k, reproduc- 

 tive pore ; /, nephridial pore; ni, anus; n and /, foot; r, cctlom ; jr, pericardium ; /, testis; 

 «, nephridium ; v, ventricle of heart ; 2/, liver. 



2d. Zone of Brachiopods, or of deep-sea corals (72—500 

 metres, 50-250 fathoms). 



3d. Zone of Nullipores, or of Corallines (27-72 metres, 

 .15-50 fathoms). 



.4th. Laminarian Zone (low tide to 27 metres, i — 15 

 fathoms). 



5th. Littoral Zone (between low and high tides). 



6th. Brackish water, sea-shores above tide, where fresh 

 and brackish waters are mixed, and where the surface may be 

 exposed to the air part of the time. 



