EVOLUTION 117 



surface of the loose sand without sinking in, and to 

 use them as fins for propelling itself through the 

 water ; while in Notavchus these two lobes are united 

 above the body in such a manner as to form a 

 muscular sac opening to the front, so that by forcibly 

 expelling the contained water the animal can dart 

 backwards. 



The " wings " of the Sea- Butterflies (Pteropods) 

 are a further modification of these pleuropodia 

 (Plate XXVL, Figs. 1-16). In the Heteropoda the 

 columellar muscle is extended through and beyond 

 the true foot, and expanded into a large and laterally 

 compressed fin (Plate XXVL, Fig. 20, /), which 

 forms an efficient swimming organ as the animal 

 progresses in an inverted position through the waters 

 of the open sea. 



The huge West Indian Fountain Shell (Strombus) 

 uses its foot in quite a different fashion. The pro- 

 podium is quite distinct, while the latter portion 

 (" metapodium ") is elongate and bears the claw- 

 shaped operculum at its extremity. When the 

 animal moves, it advances these two sections of its 

 foot, thrusts them into the silt, and then using them 

 as anchors, slides the heavy shell forward. Other 

 Strombs (Plate VII L, Fig. 3) and their kindred 

 Pterocera (the Scorpion Shells), Rostellaria, etc., 

 progress by bending the foot under the shell and 

 suddenly straightening it, thus leaping and rolling 

 over and over. 



