E have now left the headless, 

 toothless bivalves, and are 

 about to make acquaintance 

 with higher groups — Uni- 

 valves or Gasteropods, the 

 mollusks that slide on the lower surface of the foot, 

 and that have heads, eyes, and jaws. But we make 

 the transition from one class to the other through 

 a small intermediate class — univalves truly, but with 

 only a low organisation as compared with that of the 

 Gasteropoda. 



This is the order Scaphopoda or Tusk-shells, so- 

 called from the curved tubular shell that bears some 

 resemblance in miniature to the tusk of an elephant, 

 and is open at each end. The animal is of similar 

 shape to its shell, with a rudimentary eyeless head, 

 which is covered by the mantle as in the bivalves. 

 There is a projection of the pharynx, and on this is 



178 



