THE PHYLUM MOLLUSCA 



ing spiral configuration (Fig. 13.11). The apex of the shell is generally 

 closed, and the lower end presents an expanded opening, the aperture, into 

 which the head and foot may be completely retracted for protection. in 

 many species this aperture may be closed by a horny or calcareous operculum. 

 The remote ancestors of the gastropods were evidently bilaterally sym- 

 metrical animals, possibly similar in general appearance to the modern 

 amphineurans. As indicated by the anatomy and embryonic development of 



Fig. 13.11. The marine gas- 

 tropod Busycon canalicidatum in 

 positions assumed during loco- 

 motion. This large snail inhabits 

 muddy bottoms, feeding on bi- 

 valves, and reaches a length of 

 almost 1 foot. (Drawings bv 

 George T. Kline.) 



Mouth/ Vl 



/ 



Foot 



Siphon 



383 



