CLASSIFICATION OF PTEROPODA. 85 



ancestral foot. The figures also illustrate the true application of the 

 terms dorsal, ventral, anterior and posterior to the body aspects in 

 Pteropods and Cephalopods. 



It may now be useful to give a summary of orders and other 

 divisions, and in so doing, to follow, merely for the sake of con- 

 venience, the former method of separating the Pteropods as a distinct 

 Molluscan class. It must, however, be clearly borne in mind that 

 the present and more satisfactory view is to cancel entirely the 

 division Pteropoda, and to distribute its families among the Gastro- 

 poda. The name Pteropoda will thus henceforth be a term of 

 convenience used to designate a number of pelagic molluscs, in- 

 cluding animals belonging to widely separated divisions of the 

 Gastropoda. 



Pteropoda. 



Order I. ThecOSOmata ; body protected by a shell. 



Family I : HYAL.EIDJE, shell calcareous or horny, symmetric. Types 



Hyalaea (horny) ; Creseis (calcareous) ; Cleodora. 

 Family II : CYMBULINID^E, shell slipper or boat-shaped and with some 



short " arms." Types Cymbulia and Tiedemannla. 

 Family III : LIMACINIDJE. Type Spirialis, with spirally coiled shell 



having sinistral flexure, i.e. coiling in the reverse direction 



to that usually seen in Gastropod Shells. 



Order II. Gymnosomata ; body naked. 



Family I : CLIONID.E, without gills, but with short arms devoid of suckers. 



Type Clione (Clio) borealis. 

 Family II : PNEUMODERMONID.E, with gills at apex of body, and with arms 



beset with suckers. TypePneumodermon. 



Cleodora pyramidata is phosphorescent and probably others 

 are also. In Cymbulia, the small chitonous shell is internal. Spi- 

 rialis is the most closely related to the Gastropod form, its pecu- 

 liarities extending to the possession of both a spiral shell and an 

 operculum. The larvae or veligers of Cymbulia and Tiedemannia 

 also possess opercula. 



Tiedemannia, like the Cephalopods, possessed well developed 

 pigment-spots (Chromatophores) on the surface of the body, doubtless 

 a protective device. 



