33H 



THE MOLLUSKS OR SHELLFISH 



(Philodromus) running about the body of this slug, and it is also said to live 

 in the respiratory cavity, but does not appear to cause any annoyance or injury to 

 its host. 



In the genus Vitrina the animal is provided with fragile, external, horny shell, 

 not sufficiently large to receive it entirely, when contracted. The shells are all very 

 much alike, and precisely similar to those of Helicarion, another genus of Limatidce, 

 which, however, is distinguished by having the foot truncated obliquely behind, and 

 furnished with a large terminal mucus pore, and there are other anatomical differ- 

 ences. The single British species is here figured. Unlike Vitrina, the snails known 

 as Zonites and Vitrea, together with some allied genera, are wholly retractile within 

 their shells. There are several British species of Vitrea, and their identification 

 from the shells alone is a matter of considerable difficulty. In their habits they 

 closely resemble Vitrina, being carnivorous rather than vegetarian. They greedily 

 devour any kind of animal food, even in a putrid condition, and are also said to 

 prey upon some of the larger snails. They are generally found hidden away under 

 stones, dead leaves, or moss, and some of them emit a strong smell, like garlic, 

 which is perceivable at a distance of some feet. 



THE SNAIL TRIBE Family HELICIDJZ 



Like the Limacidce, this family includes forms with or without an external shell. 

 In nearly all the genera the cutting jaw of the animal is more or less ridged, and 

 not smooth as in Limax. The principal distinguishing characteristic occurs in the 

 structure of the radula, which is composed of many rows of very numerous similar 



BI<ACK SI.UG, Arion empiricorum. 

 (Natural size.) 



square-based teeth, arranged so regularly as to have a tessellated appearance. It is 

 very broad, and the number of teeth in a row, although usually less, is sometimes 

 as great or even greater than the number of rows. This family includes an enormous 

 number of species from all parts of the globe. These occur everywhere and in all 



