GASTROPODS 
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a large terminal mucus-pore, and there are other anatomical differences. 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 Helicidje. 
Like the Limac idee, 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 character occurs 
in the structure of the radula, which is composed of many rows of very numerous 
similar square-based teeth, arranged so regularly as to have a tesselated appearance. 
It is very broad, and the number of teeth in a row, although usually less, is some¬ 
times 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 climates; in dense forests, on the top of grassy downs, in valleys, fields, 
lanes, in the arid desert, and at an elevation of some ten thousand feet both in the 
Old and New World. 
To an ordinary observer, the members of the genus Arion (sometimes placed 
in a separate family, Arionidce) are merely slugs. Externally the resemblance is 
black slug, Anon cmptricorum (nat. sizej. 
very close, but the different position of the respiratory orifice, and the presence of 
a mucus-pore at the end of the foot, readily separates this genus from Limax. 
Besides these differences, the radula is of a different type. In Limax the breathing- 
hole is situated near the hinder end of the shield, whereas in Arion it is much 
