HELIX, in geometry, the same with SPIRAL, which see.
HELix, in natural history, the mail, a genus of the Vermes Testacea class and order. Animal a limax ; shell univalve, subdiaplianous, brittle ; aperture contracted, semi-lunar or roundish. Of this genus more than three hundred spe cies have been enumerated ; they are se parated into divisions ; A. whorls, 'with a carinate acute margin ; B. umbilicate, the whorls rounded ; C. rounded., imperfo rate ; D. tapering ; E. ovate, iMperforate. Of the species, we shall notice H. cornea, the shell of which above is umbilicate, flat, blackish, with four round whorls. It is found in Europe, and on the coast of Coromandel, from a single line to an inch in diameter ; shell chesnut brown, rufous, whitish, yellowish, or blueish, polished and very fine striate transversely ; whorls, four or five, rarely turned contra ry ; the inhabitant is black, with dirty grey tentacula, and produces a scarlet, but not very durable dye. H. pomatia, a snail with five spires remarkably ventri cose, slightly umbilicated, fasciated with a lighter and deeper brown : this is found in the woods of the southern counties of England ; it is said to have been introduc ed by Sir Kenelm Digby, for medical pur poses. These are confined to the south ern counties, attempts having been made, but without success, to bring them into Northamptonshire. This snail is used in many parts of Europe as food, particular ly at Rome during the weeks of Lent: here they are fattened, and grow to a very largo size. It is oviparous, very tenacious of life, and, towards winter, covers its aperture with a calcareous lid. H. horten sis, garden-snail, shell imperforate, globe= lar, pale, with broad interrupted brown bands : this species inhabits the garden and orchard in most parts of Europe ; it abounds with a viscid slimy juice, which it readily gives out by boiling in milk and water, so as to render them thick and glutinous, and the compound, especially with milk, is reckoned efficacious in con sumptive cases. Snails are very destruc tive to wall-fruit : lime and ashes sprin kled on the ground will keep them away, and destroy the young brood. Fruit, al ready bitten, should not be taken off the tree, for they will not touch the other, till they have wholly .eaten this, if left for them. The eyes of snails are lodged in
their horns, one at the end of each horn, which they can retract with pleasure. The manner of examining these eyes, which are four in number, is this : when the horns are out, cut off nimbly the extre mity of one of them, and, placing it be fore the microscope, you may discover the black spot at the end to be really a semiglobular eye.
The dissection of this animal is very cu rious ; for by this means, the microscope not only discovers the heart beating, just against the round hole near the neck, which seems the place of respiration ; but also the liver, spleen, stomach, and intes tines, with the veins, arteries, mouth, and teeth, are plainly observable. The intes tines or this creature are green, from its eating herbs, and are branched all over with fine capillary white veins ; the mouth is like a hare's or rabbit's, with four or six needle-teeth, resembling those of leeches, and of a substance like horn. Snails are all hermaphrodites, having both sexes united in each individual ; they lay their eggs with great care in the earth, and the young ones are hatched with shells completely formed. Cutting off a snail's head, a little stone appears, which is sup posed to be a great diuretic, and good in nephritic disorders : immediately under this stone, the heart is seen beating ; and the auricles are evidently distinguishable, and are membraneous, and of a white co. lour, as are also the vessels which pro ceed from them. So small an animal as the snail is not free from the plague of supporting other smaller animals on its ; and, as in other animals, we find these secondary ones either living on their surface, as lice, Etc. or only in the intestines as worms, it is very remarka ble, that the snail is infested in both these manners, lice being sometimes found on the surface of its body, and worms some times within its intestines. There is a part of the common garden snail, and of other of the like kinds, commonly called the collar; this surrounds the neck of the snail, and is considerably thick, and is the only part that is visible when the animal is retired quietly into its shell : in this state of the animal, these insects which infest it are usually seen in considerable numbers, marching about very nimbly on this part.