326 



GEOLOGICAL BIOLOGY. 



This schematic mollusk possesses "in an unexaggerated 

 form the various structural arrangements which are more or 

 less specialized, exaggerated, or even suppressed in particular 

 members of the group." It represents, as near as our knowl- 

 edge will enable us to do, the actual mollusk ancestor from 

 which the various living forms have sprung, and therefore 

 does not represent any actually living species of mollusk. 

 However, the accuracy of the schematic type is evident when 



FIG. zoo. Diagrams showing the arrangement of the organs in an ideal Mollusk. (After 

 Lankester.) a, tentacle ; 3, head ; <r, margin of mantle ; d, margin of shell ; <r, edge of body ; 

 y, edge of shell depression ; , shell ; gc, cerebral ganglion ; gpe, pedal ganglion ; gpl, plural 

 ganglion ; h, osphradium ; z, ctenidium ; k, reproductive pore ; /, nephridial pore ; m, anus ; 

 n and/, foot'; r, ccelom ; s, pericardium ; ^, testis ; #, nephridium ; v, ventricle of heart ; 

 zl t liver. 



we attempt to compare with it a living specimen of some one 

 species of mollusk. 



It is an, attempt to give form and definite relation to the 



terms of a systematic definition of the characters of the 

 branch Mollusca. In his diagrams of a series of mollusks the 



same method is used to give formative expression to the 

 characteristics of the several classes. 



