=4e Ss 
The introduction of an animal, such as a strange snail, whose habits, fecun- 
dity, dearth of parasites or predators, are completely foreign to the com- 
munity, may have rather profound and adverse effects on the organization of 
the communities and nearly every organism init. The immigrant snails, in 
building up a large population, may not only out-compete the native well- 
regulated snails and other herbivores for food, but also by destroying vege- 
tation deprive many organisms of essential shelter. Reduction in abundance 
or disappearance of the native herbivores results in the reduction or extinc- 
tion of many carnivorous species, which may then affect other members of 
the community, either by relieving a check in their increase or depriving 
them of a livelihood. This can cause many beneficial animals and plants to 
permanently disappear from the area. Therefore, the indirect economic 
import of introduced species may actually be far greater than the direct and 
more obvious economic importance. 
HABITS OF SNAILS AND SLUGS 
Land Snails and Slugs 
Native land snails and slugs may be found almost everywhere, but in gen- 
eral prefer habitats offering shelter, adequate moisture, an abundant food 
supply, and for snails, generally an available source of lime. Forested 
river valleys most generally supply such habitats, and those with outcrops 
of limestone usually show the most abundant and varied mollusk faunas. In- 
troduced species often tend to be somewhat more urban, For example, in 
their native Europe the slugs Limax marginatus’ and Milax gagates live in 
woodlands and on damp rocks on open hillsides and are not slugs of culti- 
vated areas, But in the United States they are mostly found around dwell- 
ings and in greenhouses and gardens, 
Land snails and slugs are mainly nocturnal, but following a rain may come 
out of their hiding places during the day. Temperature and moisture are 
the main factors to account for their nocturnal habits, and not the presence 
or darkness per se, During the day they may be found resting under old 
boards and logs, under bricks and stones, in rock piles and cellars, among 
hedge rows, and beneath damp refuse and litter. Snails are more adaptable 
to unfavorable environmental conditions, such as drought, because they can 
cover the aperture of their shells with a mucous sheet, the epiphragm, which 
hardens and thereby prevents desiccation. Some snails have been known to 
remain in this dormant state for years, only to come out and resume activ- 
ity when they are moistened. 
Most land snails and slugs pass the winter in sheltered places under stones, 
logs, and boards, or buried inthe ground. Some snails, Theba pisana, for 
example, are not so secretive, aestivating in the open on such objects as 
tree trunks and fence posts during periods of unfavorable humidity and tem- 
perature. 
