But there are a considerable number of snails, both terrestrial and aquatic, which are not Pulmonates.
For the morphology and classification of snails, see Gastropoda.
The food of the adult is almost exclusively animal, - insects, especially large ants, snails, lizards and snakes, but it also eats certain large red berries.
Snails are reared in some parts of the country as an article of food, those of Burgundy being specially esteemed.
They live for some time in water or mud, occasionally entering the bodies of water snails, but undergo no change until they reach the lung of a frog, when the cycle begins anew.
Although several species belonging to the second class occasionally enter the bodies of water snails and other animals before reaching their definitive host, they undergo no alteration of form in this intermediate host; the case is different, however, in Filaria medinensis and other forms, in which a free larval is followed by a parasitic existence in two distinct hosts, all the changes being accompanied by a metamorphosis.
The so-called eelworms (Nematodes) may do immense damage on roots and in the grains of cereals, and every one knows how predatory slugs and snails are.
The spores of Rusts, Erysipheae an d other Fungi may be conveyed from plant to plant by snails; those of tree-killing polyporei, &c., by mice, rabbits, rats, &c., which rub their fur against the hymenophores.
The very large assemblage of forms coming under this order comprises the most highly developed predaceous sea-snails, numerous vegetarian species, a considerable number of freshwater and some terrestrial forms. The partial dissection of a male specimen of the common periwinkle, Littorina littoralis, drawn in fig.
The surface x of the mantle between the rectum and the gill-plume is thrown into folds which in many sea-snails (whelks or Buccinidae, &c.) are very strongly developed.
In some Pulmonata (snails) the foot is extended at right angles to the visceral hump, which rises from it in the form of a coil as in Streptoneura; in others the visceral hump is not elevated, but is extended with the foot, and the shell is small or absent (slugs).
Some members of the Argyopidae (Cyclosa) are exactly like small snails; others (Cyrtarachne) resemble Coccinellidae in shape and colour.
Now, Coccinellidae (ladybirds) are known to be highly distasteful to most insectivorous mammals and birds, and snails would be quite unfit food for the Pompilid or Ichneumonid larvae, so that the reason for the mimicry in these cases is also perfectly clear.
Marine Gasteropods are occasionally termed "sea-snails," and the compounds "pond-snails," "river-snails," "water-snails" are in common use.
The commonest land-snails are those species which constitute the family Helicidae, order Pulmonata, sub-order Stylommatophora.
The term "water-snails" includes the whole of the remaining sub-order of the Pulmonata, namely, the Basommatophora, in which the eyes are sessile, with the exception of the Auriculidae.
Thus the whole of the Pulmonata (which breathe air, are destitute of gill-plumes and operculum and have a complicated hermaphrodite reproductive system) are either snails or slugs.
The land-snails which have no gill-plume in the mantle-chamber and breathe air, but have the sexes separated, and possess an operculum, belong to the orders Aspidobranchia and Pectinibranchia, and constitute the families Helicinidae, Proserpinidae, Hydrocenidae, Cyclophoridae, Cyclostomatidae and Aciculidae.
The fresh-water snails which are not Pulmonates are the Paludinidae, Valvatidae and Ampullaridae, together with Neritina, a genus of the Neritidae.
The species of Helix are all herbivorous, like the Pulmonata generally; snails and slugs are well-known enemies to the gardener.
The blackbird feeds chiefly on fruits, worms, the larvae of insects and snails, extracting the last from their shells by dexterously chipping them on stones; and though it is generally regarded as an enemy of the garden, it is probable that the amount of damage by it to the fruit is largely compensated for by its undoubted services as a vermin-killer.
It is about a foot in length, lives on snails and worms and is provided with both lungs and gills.
Ross wrote: "So may he (Sir Thomas Browne) doubt whether in cheese and timber worms are generated; or if beetles and wasps in cows' dung; or if butterflies, locusts, grasshoppers, shell-fish, snails, eels, and such like, be procreated of putrefied matter, which is apt to receive the form of that creature to which it is by formative power disposed.
In the Nova Scotian tree trunks land snails (Archaeozonites, Dendropupa) have been found.
Land-snails, mostly Achatinellidae, are remarkably frequent and diverse; over 300 varieties exist.
L * An independent anatomical investigation of the Mollusca had been carried on by the remarkable Neapolitan naturalist Poli (1791), whose researches 2 were not published until after his death (1817), and were followed by the beautiful works of another Neapolitan zoologist, the illustrious Delle Chiaje.3 The embranchement or sub-kingdom Mollusca, as defined by Cuvier, included the following classes of shellfish: (1) the cuttles or poulps, under the name Cephalopoda; (2) the snails, whelks and slugs, both terrestrial and marine, under the name Gastropoda; (3) the sea-butterflies or winged-snails, under the name Pteropoda; (4) the clams, mussels and oysters, under the name Acephala; (5) the lamp-shells, under the name Brachiopoda; (6) the seasquirts or ascidians, under the name Nuda; and (7) the barnacles and sea-acorns, under the name Cirrhopoda.
In small flowers which are crowded at the same level or in flat flowers in which the stigmas and anthers project but little, slugs or snails creeping over their surface may transfer to the stigma the pollen which clings to the slimy foot.
Besides valuable contingents of the celebrated Balearic slingers, the Romans derived from their new conquest mules (from Minorca), edible snails, sinope and pitch.