THE MOLLUSCA 



105 



soft parts. One species called the " oyster drill" destroys 

 large numbers of oysters in this way. 



The chitons which are allied to the gastropods have a 

 broad, creeping foot and a dorsal shell composed of a row 

 of eight pieces. They are often found on 

 rocks at low tide. The Cephalopoda differ 

 greatly in appearance from the other mol- 

 lusca. Their name is derived from the 

 fact that the foot is produced into a num- 

 ber of arms (8 or 10) surrounding the head. 

 In most cephalopods these arms are fur- 

 nished with rows of suckers which are used 

 for retaining hold of objects. The cepha- 

 lopods were represented in former ages of 

 the earth's history by vast numbers of 

 varied forms. Some of these, the ammo- 

 nites, had a coiled, chambered shell which 

 was often beautifully sculptured. The or- 

 thoceratites had a straight, chambered shell which in some 

 species reached a length of fifteen feet. Of the forms now 

 living within a shell, the chambered nautilus is the only 



FIG. 89. Tere- 

 bra babylonia. 



FIG. 90. Limpets, a key hole limpet at the left. 



survivor. It is found in deep waters in the Pacific and 

 Indian Oceans, but its much prized shell is sometimes cast 

 up on the shore. In the so-called paper nautilus there is 

 a thin shell which is formed, not by the mantle, as in the 



