OTHER SOIL ANIMALS - The Soil - page 110

crop rotation, and by the use of resistant varieties of potatoes and other susceptible
crops. Crop resistance can take several forms: the varieties may not be invaded by eel-
worm larvae at all, or else the larvae fail to develop normally. Biologically, the most
interesting way of frustrating nature is that in which the plant allows only male larvae
to develop fully!
S NAILS AND SLUGS
There are nineteen families of land snails in the British Isles and four families of slugs
comprising some 87 and 30 species respectively. Of these, fewer than a dozen are fa-
miliar to the layman, and then mainly because of their unwelcome attentions to crops
and vegetables. If, however, one were to examine carefully a few shovels of litter
and soil from a beech woodland or limestone grassland, shaken through a sieve and
spread out over a tray, one would discover a great variety of small snails, “inexhaust-
ibly variegated in their marblings and convolutions” to quote Teilhard de Chardin.
Nor should it be assumed that these are just young individuals; most snails are less
than a centimetre along the major shell axis, and the ubiquitous Carychium minimum
is only 1.9mm in height when fully grown.
There are about 20 species that are restricted to calcareous soils, notably the
chrysalis snail Abida secale which frequents dry, stony ground in the Cotswolds and
elsewhere in south and west England, and the burrowing Pomatias elegans which
lives in loose soil and moist leaf mould in chalk and limestone districts. In this coun-
try, the Roman or edible snail Helix pomatia is confined to hillsides, banks and quar-
ries on calcareous soils. It was almost certainly introduced by man as a culinary del-
icacy as it is absent from Pleistocene fossil deposits, and the earliest record is from
a 4th century Roman excavation. This species must hold the record for longevity as
well as size. G. Lundqvist, in Sweden, kept notes on one particular individual in his
garden for 30 years, and recorded a weight of 67 grammes - as much as a large hen's
egg.