ALLIES OF THE PELECYPODA : MOLLUSCA 185 



periwinkle at the left of the picture, with the oyster-drill of 

 the foreground engaged in devouring the soft parts of its 

 victim through the hole made in the thick shell. For the 

 purposes of illustration the oyster-drill was placed on another 

 periwinkle in the same position it occupied on the first when 

 discovered. In deep water the oyster-drill devotes itself to 

 the business which gave it the name it bears. There are 

 several other species of snails, however, which have the same 

 habit of boring through the oyster's valves and feeding on 

 the soft parts. 



The oyster-drill has organs not found in the snails already 

 described in this chapter. In a little groove in the "mouth" 

 of the shell lies a small tube called the siphon, which carries 

 water to a gill-chamber, where the blood is supplied with 

 oxygen. Another organ is a horny plate growing on top of 

 the posterior end of the foot. When the foot is withdrawn 

 into the shell, the horny plate, which is called the operculum 

 (a lid, or cover), is drawn into the mouth of the shell. The 

 operculum is of great service to the animal in keeping out 

 unwelcome visitors. 



The oyster-drill, in common with all other species of snails 

 that have their habitat on the seashore, possesses a very thick 

 shell, adapted to the severe conditions of life there. The 

 sand and pebbles are constantly shifting under the pressure 

 of the waves, and only those snails that have thick shells can 

 endure the conditions. 



The Periwinkle. The periwinkle, a littoral (shore) species 

 (Littori'na litto'rea, Fig. 99), is a native of Europe. Its pres- 

 ence here is accounted for by the supposition that specimens 

 were accidentally thrown in with the gravel used for ballast 

 on ocean-going vessels, and thrown out again when the vessel 

 reached its port in America. The first specimens noticed in 

 America by conchologists (students of shells) were reported at 

 Halifax, Nova Scotia, in 1857. Since that time the periwinkle 



