2r6 



AN INTRODUCTION TO ZOOLOGY. 



the foot, and the anterior edge of the mantle. The 

 Teredo, or Ship-worm, which burrows in timber, be- 

 longs to the same family ; it is much, elongated and 

 modified in shape, to meet the requirements of its 

 mode of existence. Many of the Siphoniata, for 



Fig. 68. — Shell of a Sipl oniate Mo11ubc» the Gaper Shell (Ifya frimcata), so 

 called on account o£ the open end of the slioll, which admits the large siplioii&. 



example the Razor-shell and the Gaper, make a deep 

 vertical burrow in the sand, and push their long 

 siphons up to the surface to breathe. 



A different classification of the Lamellibranchiata 

 is founded on the nature of the adductor muscles, 

 which draw the shell together, and leave marks on 



