The royal natural history . ,—Family Helicid^e. Like the Limacidce, this family includes forms with or without an externalshell. In nearly all the genera the cutting-jaw of the animal is more or lessridged, and not smooth as in Limax. The principal distinguishing character occursin the structure of the radula, which is composed of many rows of very numeroussimilar square-based teeth, arranged so regularly as to have a tesselated 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 inclu
The royal natural history . ,—Family Helicid^e. Like the Limacidce, this family includes forms with or without an externalshell. In nearly all the genera the cutting-jaw of the animal is more or lessridged, and not smooth as in Limax. The principal distinguishing character occursin the structure of the radula, which is composed of many rows of very numeroussimilar square-based teeth, arranged so regularly as to have a tesselated 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 anenormous number of species from all parts of the globe. These occur everywhereand 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 theOld and New World. To an ordinary observer, the members of the genus Arion (sometimes placedin a separate family, Arionida) are merely slugs. Externally the resemblance is. black sluo, Anon empi/ricorum (nat. sizej. very close, but the different position of the respiratory orifice, and the presence ofa mucus-pore at the end of the foot, readily separates this genus from 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 Avion it is much 346 MOLLUSCS. further forward. In the present genus there is no internal shield-like shell, asin Limax, but this is represented by a few unequal calcareous particles beneaththe mantle. Nine species of this genus are said to occur in Britain, and of thesethe large A. empiricorum is the commonest and best known. It is sometimesintensely black, but it may be brown, red, yellow, greenish, or even white. Thisgreat variation in colour is unaccountable, for black and red specimens occur inthe same districts where the natural surroundings are practically the same. Theedge of the foot, when crawling, d
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Keywords: ., bookcentury1800, booksubjectnaturalhistory, booksubjectzoology
