ALLIES OF THE ACEPHALA 213 



unfortunate 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, 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. 119), 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 ves- 

 sel reached its port in America. The first specimens noticed 

 in America by conchologists (students of shells) were re- 

 ported at Halifax, Nova Scotia, in 1857. Since that time 

 the periwinkle has been making its way down the coast. It 

 may now be found in southern waters. The new home in 

 America seems to be especially favorable for the periwinkle, 

 for on the rocky coast of New England there are so many 

 of them that on first impression there appears to be no other 

 species. This snail depends on plant food altogether. 



On account of its great abundance the periwinkle is much 

 used as an article of human food in England and on the 

 Continent. It is one of the many species of snails and slugs 

 used as food, especially by the French people. 



