EVOLUTION 17 
surface of the loose sand without sinking in, and to 
use them as fins for propelling itself through the 
water; while in Notarchus 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 XXVI., 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 XXVI., 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 VIII., 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. 
