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. 



b 



FIG. 35. Schematic Mollusk. (After Lankester.) a, tentacle ; 3, head ; c, margin of mangle ; </, 

 margin of shell ; e, edge of body ; /, edge of shell depression ; g, shell ; %c. cerebral gan- 

 glion; gj>e, pedal ganglion; gpl, i leural ganglion ; h, osphradium ; /', ctenidium ; , reproduc- 

 tive pore ; /, nephridial pore; 7, anus; n and p, foot; r, coelom; j, pericardium ; t, testis ; 

 , nephridium ; z/, ventricle of heart ; z/, 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, 1-15 

 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. 



