246 GEOLOGICAL BIOLOGY. 



the mature individuals, we turn to the synthetic classification 

 of the same organisms as presented by Lankester, in which 

 the distinguishing points are chiefly found in those characters, 

 by which resemblance or relationship to some other different 

 organism is traced. 



In the first case differences and in the second case resem- 

 blances form the chief criteria upon which the classification is 

 based. 



Lankester's Classification of the MoUusca. — According to Pro- 

 fessor Lankester,^ whose classification is one of the most rad- 

 ical and modern, the branch Mollusca includes four classes, 

 divided into two groups: Class i, Gastropoda; Class 2, Sca- 

 phopoda; Class 3, Cephalopoda; Class 4, Lamellibranchia. 



The Coelomata. — Among the Metazoa, to which he applied 

 the name Enterozoa in distinction from Protozoa, Lankester 

 recognized two fundamental divisions: (A) the Coelentera, in 

 which the enteron or digestive cavity communicates directly 

 and is continuous with the coelom or body cavity; (B), the 

 Coelomata, including the Mollusca and higher invertebrates, 

 in which the enteron is separate from the coelom which sur- 

 rounds it and with which it communicates through its tissues 

 by osmosis. The products of digestion thus transmitted 

 into the coelom or body cavity are distributed through a 

 system of canals and caused to circulate by a contractile 

 organ which in its more differentiated condition is the heart. 

 The special advance in differentiation in the Coelomata con- 

 sists in the separation of the alimentary cavity into a distinct 

 digestive cavity and an assimilative cavity, the circulative 

 and purificative functions being auxiliary to the general as- 

 similative, as distinct from the digestive, functions. 



Description of the Mollusca. — The Mollusca are typically 

 Coelomata. They have also, in common with the other Coe- 

 lomata, a region in front of the mouth developed as the ex- 

 pression of the specialized function oiforivard, as distinct from 

 rotatory, motion; and in this region, which Lankester calls 

 the prostoiniiim, are differentiated, when present, the chief 

 organs of sense. As to body form, the Mollusca have differ- 



*See article Mollusca, " Encyclopaedia Britannica," gth Edition, vol. xvi. 



