THE PHYLUM MOLLUSCA 



snails are apparently descendants of land snails which returned to an aquatic 

 habitat. Other descendants of the primitive, creeping marine gastropods took 

 to a pelagic life in the open ocean and gave rise to the existing pteropods 

 and heteropods. 



THE CLASS SCAPHOPODA 



The mollusks belonging to this class are characterized by dorsoventral 

 elongation, without spiral coiling, and by a simple, unbranched foot adapted 

 for burrowing. The body is covered by a shell which is slightly curved and 

 resembles a miniature tusk. The shell is open at both ends; the foot, together 

 with several oral tentacles, protrudes from the ventral end, which also con- 

 tains the incurrent opening into the mantle cavity. The dorsal end bears the 



Shell 



Mantle cavity 



Digestive 

 gland 



Nephridium 



Stomach 



Visceral 

 ganglion 



Cephalic 

 ganglion 



Anus 



Cirri 



Statocyst 



Fig. 13.15. Class Scaphopoda: Denlalium en/ale. A, photoe;raph of a preserxed specimen, lateral 

 asjK-ct. B. semidiae;rammatir lonsjitudinal section to show general internal organization. (.-1, 

 photograph by Bassett Maguire. Jr.; B, redrawn from W. Stempell, 1926, ~oolooie im (iriwdriss.) 



387 



