in geometry, a spiral line. See Spiral.

The word is Greek, ἑλίξ, and literally signifies "a wreath or winding;" of ἑλίξις ἐνεργεία, "I environ."

In architecture, some authors make a difference between the helix and the spiral. A flail-case, according to Daviler, is in a helix, or is helical, when the stairs or steps wind round a cylindrical newel; whereas the spiral winds round a cone, and is continually approaching nearer and nearer its axis.

Helix is also applied, in architecture, to the caulicles or little volutes under the flowers of the Corinthian capital; called also urille.

anatomy, is the whole circuit or extent of the auricle or border of the ear outwards. In opposition to which, the inner protuberance surrounded thereby, and answering thereto, is called anthelix. See Anatomy, n°141.

Snail, in zoology, a genus belonging to the order of vermes testacea. The shell consists of one spiral, brittle, and almost diaphanous valve; and the aperture is narrow. There are 60 species, principally distinguished by the figure of their shells. They are of various sizes, from that of a small apple to less than half a pea. Some of them live on land, frequenting woods and gardens, or inhabiting clefts of rocks and dry sand-banks. Others of them are aquatic, inhabiting ponds, deep rivers, and the ocean. The principal species are,

1. The janthina, with a violet-coloured shell, is remarkable for the extreme thinness of its texture, which breaks with the least pressure, and seems therefore entirely calculated to keep the open sea, or at least to shun rocky shores. It inhabits the seas of Europe, especially the Mediterranean; those of Asia and Africa; and also the ocean. The living animal, when touched, exudes a juice which stains the hands of a violet colour. Dr Hawkeworth, in his account of Cooke's voyage, mistakes this shell for that which yielded the purpura of the ancients. But whoever looks into Pliny, can never have the least idea that the thin shell aforementioned could be the same with it. They had several shells which yielded the purple dye; but these were all rock-shells *, and very different both in figure and shape from the little helix janthina; which is not cinum and calculated for the neighbourhood of rocks, as already Murex mentioned. VId. Plin. lib. v. cap. i. and lib. ix. cap. 60, 61. See also Don Ant. Ulloa's Voyage to South America. book iv. ch. 8.

2. The pomatia, or exotic snail, with five spires, most remarkably ventricose, and fasciated with a lighter and a deeper brown, is a native of France, where it inhabits the woods; but has been naturalized in England, where it inhabits the woods of the southern counties. It was introduced, as it is said, by Sir Kenelm Digby; whether for medical purposes, or as food, is uncertain: tradition says, that to cure his beloved wife of a decay was the object. They are quite confined to our southern counties. An attempt was made to bring them into Northamptonshire, but they would not live there.—These are used as a food in several parts of Europe during Lent; and are preserved in an escargatoire, or a large place boarded in, with a floor covered half a foot deep with herbs, in which the snails nestle and fatten †.—They were also a favourite dish ‡ with the Romans, who had their cochlearia, a nursery similar to the above. Fulvius Hirpinus § was the first Pliny, l.x. inventor of this luxury, a little before the civil wars between Caesar and Pompey. The snails were fed with bran and sodden wine. If we could credit Varro ||, || L. iii., they grew too large, that the shells of some would hold ten quarts! People need not admire the temperance of the supper of the younger Pliny $, which consisted § Epift. xv. of only a lettuce a-piece, three snails, two eggs, a barley cake, sweet wine and snow,—in case his snails bore any proportion in size to those of Hirpinus.—Its name is derived not from anything relating to an orchard, but from ῥοῦσα, an operculum, it having a very strong one. This seems to be the species described by Pliny, Pliny, lib. viii, c. 39, which he says was scarce; that it covered itself with the opercle, and lodged under ground; and that they were at first found only about the maritime Alps, and more lately near Velitra. [See Plate CCXXXIV. the figure half the natural size.]

3. The hortensia, or garden-snail, is in form like the last, but less, and not umbilicated and clouded, or mottled with browns. It abounds with a viscid slimy juice, which it readily gives out by boiling in milk or water, so as to render them thick and glutinous. The decoctions in milk are apparently very nutritious and demulcent, and have been recommended in a thin acrimonious state of the humours, in consumptive cases and emaciations.

The eyes of snails are lodged in their horns, one at the end of each horn, which they can retract at pleasure. The manner of examining these eyes, which are four in number, is this: when the horns are out, cut off nimbly the extremity of one of them; and placing it before the microscope, you may discover the black spot at the end to be really a semiglobular eye.—The dissection of this animal is very curious; 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 intestines, with the veins, arteries, mouth, and teeth, are plainly observable. The guts of this creature are green, from its eating of 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 subflance 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 supposed to be a great diuretic, and good in all nephritic disorders. Immediately under this stone the heart is seen beating; and the auricles are evidently distinguishable, and are membranous, and of a white colour; as are also the vessels which proceed from them.

Snails discharge their excrements at a hole in their neck; they also breathe by this hole, and their parts of generation are situated very near it. The penis is very long, and in shape resembles that of a whale. In the process of generation, it has been observed, that with the male and female part there issues, at the aperture of the neck, a kind of spear, shaped like the head of a lance, and terminating in a very acute point; and when the two snails turn the clefs in their necks towards each other, the spear issuing from one pricks the other, and then either drops to the ground or is carried off by the snail it has pricked. This snail instantly withdraws, but soon after rejoins the other, which it pricks in its turn; and after such mutual puncture, the copulation never fails of being consummated. Snails are said to couple three times at the distance of about fifteen days from each other, nature producing a new spear for each time of copulation, which lasts ten or twelve hours. At the end of about eighteen days they bring forth their eggs by the aperture of their neck.

So small an animal as the snail is not free from the plague of supporting other smaller animals on its body; and as in other animals we find these secondary ones either living only on their surface, as lice, &c. or only in the intestines, as worms, it is very remarkable that this creature infects the snail in both these manners; being found sometimes on the surface of its body and sometimes 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 infect it are usually seen in considerable numbers marching about very nimbly on this part: besides, the snail, every time it has occasion to open its anus, gives them a place by which to enter into its intestines, and they often seize the opportunity.

Snails are great destroyers of fruit in our gardens, especially the better sorts of wall-fruit. Lime and ashes sprinkled on the ground where they most resort will drive them away, and destroy the young brood of them: it is a common practice to pull off the fruit they have bitten; but this should never be done, for they will eat no other till they have wholly eaten up this if it be left for them.