PHYLOGENESIS IN CLASSIFICATION. 25 1 



beset with hooks or suckers, and the mid-foot is developed 

 into a tube either closed or with lapping edges (No. 6). 



Distinctive Features of the Lankester Classification. — The 

 distinctive feature of Lankester's classification is seen in his 

 descriptions of the subclasses. To show the nature of the 

 characters selected as definitive of the divisions recognized, 

 the chief characters of the subclass (2), Gastropoda Aniso- 

 pleura, will be quoted, and for any further details the reader 

 is referred to the fully elaborated and illustrated article in 

 the " Encyclopaedia Britannica '' on Mollusca. 



The definition includes the following characters, viz. : 



"Gastropoda, in which, whilst the head and foot retain 

 the bilateral symmetry of the archi- Mollusca, the visceral 

 dome, including the mantle-flap dependent from it, and the 

 region on which are placed the ctenidia, anus, generative and 

 nephridial apertures, have been subjected to a rotation tend- 

 ing to bring the anus from its posterior median position, by 

 a movement along the right side, forwards to a position above 

 the right side of the animal's neck, or even to the middle 

 line above the neck. . . . The shell is not a plate enclosed 

 in a shell-sac, but the primitive shell-sac appears and dis- 

 appears in the course of embryonic development, and a 

 relatively large nautiloid shell (with rare exceptions) develops 

 over the whole surface of the visceral hump and mantle skirt. . . 



" The shell and visceral hump in the Anisopleura incline 

 normally to the right side of the animal. . . . Atrophy of the 

 representatives on one side of the body of paired organs is 

 very usual." (p. 644.) 



In these descriptions it will be noticed that characters 

 chosen as distinctive are based upon comparison of the type 

 under description with forms from which it is supposed to 

 have been developed embryologically, or from which it is 

 supposed to have descended by evolution. 



The Gastropoda Anisopleura is conceived of as a Gastro- 

 pod mollusk which has become modified in a particular way 

 in the course of evolution. 



