Genus of gastropods
Helix is a genus of large, air-breathing land snails native to the western Palaearctic and characterized by a globular shell.
It is the type genus of the family Helicidae, and one of the animal genera described by Carl Linnaeus at the dawn of the zoological nomenclature.
Members of the genus first appeared in the fossil record during the Miocene.
Well-known species include "Helix pomatia" (Roman snail, Burgundy snail, or edible snail) and "Helix lucorum" (Turkish snail). "Cornu aspersum" (garden snail), though externally similar and long classified as a member of "Helix" (as "Helix aspersa"), is not closely related to "Helix" and belongs to a different tribe of Helicinae.
Taxonomy.
In Linnaeus' 10th edition of Systema Naturae, which marks the beginning of the zoological nomenclature, the generic name "Helix" had been used for a variety of terrestrial (e.g. "Zonites algirus"), freshwater (e.g. "Lymnaea stagnalis"), and marine (e.g. "Fossarus ambiguus") gastropods. Later authors restricted the name's use to stylommatophoran species with flattened to globular shells, including zonitids and other groups. In the course of the 1800s, several thousand species of Europe and abroad have been described in "Helix". By the early 1900s, the genus was split into many separate genera, leaving only species closely related to its type species "Helix pomatia" in the genus. However, due to the previously broad concept of the genus, "Helix" is part of the original combination (basionym) of many gastropod names and there still are many nominal taxa described in "Helix" whose generic placement remains unresolved (taxa inquirenda), although they clearly do not refer to any species of "Helix" in its present sense.
Since the 2000s, "Helix" has been subject to extensive molecular phylogenetic studies and taxonomic revisions. These led to the exclusion of several species, most notably the garden snail, and inclusion of others ("H. ceratina", "H. nicaeensis"). "Maltzanella", for long considered a subgenus of "Helix", was also formally removed from the genus, but is the sister group of "Helix".
Two subgenera are currently recognized:
Description.
"Helix" comprises large land snails species, with shell diameter of 2–6 cm. The shell is globular to conical, with five darker bands that may be variably reduced or fuse together. The globular shell distinguishes "Helix" from most of the related genera (tribe Helicini), except for "Maltzanella" and "Lindholmia". The surface has a structure of fine transversal ribs, developed to a varying degree, and there may be very fine spiral grooves as well. The shell is never malleated. Colour of the foot varies. It may be grey, brown, black or pink; the back of the foot is dark in several species.
The shells of "Helix" species are dextral. Sinistral individuals are very rare, but are occasionally found (e.g. "H. pomatia", "H. thessalica" and "H. lutescens").
Characters on the genital system have been used to define the genus and its subgenera. Unlike "Cornu", the penis of "Helix" contains two papillae with a central opening. There appears to be a tendency for a shortening of the diverticulum of bursa copulatrix and of the eppiphallus, but there is an overlap with related genera in these characters. Mucous glands adjoining the dart sac are usually richly branched.
Distribution.
"Helix" is a western Palaearctic genus. The species diversity is concentrated to the Balkans and Anatolia, with the greatest phylogenetic diversity in Greece. The natural western distribution limits run through mainland France ("Helix pomatia"), Corsica ("Helix ceratina"), and Algeria ("Helix melanostoma"). In the north, the natural distribution of "H. pomatia" reaches central Germany and the southern margins of the North European plain. The southernmost species live in North Africa ("H. melanostoma", "H. pronuba") and the southern Levant ("H. engaddensis"). The eastern limits are reached in western Iran and Iraqi Kurdistan ("H. salomonica") and in the Caucasus ("H. lucorum"); "H. thessalica" reaches through Ukraine at least to the western Russian frontier.
Genetics.
Haploid genome size was estimated to be nearly 4 Gbp (C-value 4 pg) with a GC-content of ~42%, but it is unclear which species was studied due to a discrepancy between the stated species and sample origin. The haploid number of chromosomes is 27 (studied species were "H. lucorum", "H. buchii", "H. pomatia", "H. gussoneana" and "H. straminea"). In "H. pomatia", all chromosomes have median or sub-median centromeres. Small supernumerary chromosomes were reported from "H. pomatia" from England.
The mitochondrial genome of "H. pomatia" is available (ca. 14 070 bp long).
Genital system.
The structure of the genital system corresponds in most aspects to that of other Helicidae. Its anatomy and function have been studied in detail in "H. pomatia".
As all stylommatophorans, "Helix" snails are hermaphrodites. Sperms and egg cells are produced in a common gonad, the ovotestis (hermaphroditic gland), which is embedded in the hepatopancreas (digestive gland) near the apex of the shell. Gametes are transported through a hermaphroditic duct (ovotestis duct) to the fertilization pouch–spermatheca complex (carrefour) embedded at the base of the albumen gland. In this organ, the foreign sperm (sperm from the other individual) is stored in spermathecal sacs (receptacula seminis) and egg fertilization takes place in the fertilisation pouch. The albumen gland provides nourishment for the developing fertilized eggs, and its size greatly varies with the stage of the reproductive cycle. The snail's own sperm and fertilized eggs are transported by specialised regions of the spermoviduct (sperm groove and uterus), which distally separate into a male (vas deferens) and female (free oviduct) parts of the genital system.
The male genitalia consist of a tube that serves the formation of a spermatophore and its transfer into the female parts of the mating partner. The penis is the most distal and muscular part. A spermatophore is formed in the epiphallus (between vas deferens and penis) and the flagellum (continuation of the epiphallus proximally from the vas deferens opening); the latter forms the tail of the spermatophore. During copulation, the penis everts (like a sleeve turned inside out) and is in this process inserted in the vagina. A retractor muscle attaches on the epiphallus and retracts the male genitalia after copulation.
The female part consists of a vagina (sometimes called the copulatory canal) and the bursa copulatrix (gametolytic gland) with its stalk and usually a diverticulum of the stalk. The vagina serves the transport of the foreign spermatophore and of eggs. The bursa is attached by a thin stalk to the vagina (marking the boundary between vagina and the free oviduct). The stalk in most cases bears a diverticulum, a blind tube that receives the front part of the spermatophore if present. The diverticulum has been proposed to be a remnant of a seminal duct that originally transported foreign sperm into the fertilization pouch. Sperm leave the tail of the spermatophore and migrate into the oviduct and then to the fertilization pouch; the vast majority of the sperm does not escape in this way and is digested in the bursa. In "Helix", there is tendency for a reduction of the diverticulum, and it can be missing in several species.
Dart apparatus, although positioned on the vagina, is functionally also part of the male genitalia. It is composed of a single muscular dart sac and two mucous glands (digitiform or accessory glands) on its sides. The mucous glands are branched at their base; the number of branches varies between "Helix" species. The single dart is aragonitic, straight or only weakly curved, with four blades (vanes) along its length and a corona at its base. The dart apparatus is missing in "Helix salomonica". The mucous glands produce mucus that covers the dart during shooting and is thereby injected into the body of the partner, where it induces shortening of the diverticulum and peristaltic movements of the bursa stalk that help the foreign sperm to escape lysis in the bursa.
The whole genital system forms from a single tubular invagination of the ectoderm. During the development, folds form along the internal space of the tube than eventually completely separte first vas deferens and penis, then also bursa copulatrix. The male and female parts open into a common atrium and a genital pore positioned ventrally behind the right optic tentacle.
A rare teratological individual with paired male genitalia (penis, epiphallus, flagellum) has been reported from Germany.
Reproduction.
The aspects of reproduction have been studied primarily in "H. pomatia", with limited information from other species.
Mating behaviour has been described several times for "H. pomatia". In the initial phase, the two snails raise their feet and press the soles against each other and touch each other's tentacles and mouthparts. This takes 15–30 min. Some time later, the dart shooting takes place, although many matings progress without a love dart being employed. The mucous glands produce a whitish secretion just before the shooting, that contains hormones promoting the compound that improves preservation of foreign sperm in the receiving individual. Then, again after a pause, comes the copulation, usually preceded by several unsuccessful attempts in which the reciprocal insertion of penes is not achieved and the genital organs are partially retracted back into the body. Finally, both individuals simultaneously insert the penes into each other's female opening. Within ca. 4–7 min the spermatophore is formed and transferred, after which the snail disengage and retract the everted genital organs. However, the complete reception of the spermatophore takes another 2–3 hours, during which the snails remain partially retracted and inactive.
It has been reported that only one spermatophore is usually transferred during copulation in "H. pomatia", so one animal functions as a male and one as a female in each mating. According to that report it is mostly the older snail who lay eggs, while younger function as males.
In "H. pomatia", mating takes place mostly from May to June, but often continues more sporadically up to the autumn. However, because activity is dependent on climatic conditions, the timing of mating and egg laying differs in some other species.
In "H. pomatia", the snails copulate usually with multiple mates. Received foreign sperm may be stored for more than a year before fertilization. Eggs are laid into a chamber dug in the soil by the parent 4–6 days after mating. The eggs are formed only as the nest is built. As in other pulmonates, the eggs are rich in galactogen produced by the albumen gland. The eggshell is partially mineralized, with crystals of calcium cabonate in a flexible membrane. Clutch size is given in the literature within the range 3–93. Hatching follows roughly 25–26 (range 18–31) days after egg laying, but the snails remain additional 8–10 days in the nest. An individual may lay more than one clutch per season. The clutch size may be different in other species. Mean clutch size in "H. lucorum" is similar to that of "H. pomatia", while the range reported for "H. albescens" (smaller body size than "H. pomatia", with larger eggs) is only 7–22 eggs per clutch.
The sperm morphology follows the basic pattern known from "pulmonates". Mitochondria are fused and form a continuous sheath around the flagellum. Large part of mitochondrial derivative is made up by a proteinaceous paracrystalline structure, in which there is a glycogen-filled canal. The canal runs helically along the flagellum, forming a so-called glycogen helix. There is only a single, loosely coiled glycogen helix in "Helix".
Life history.
In "H. pomatia", sexual maturity is reached after 2–4 (–6) overwinterings, i.e. at the age of 2–4 (–6) years, but this differs between localities as well as between snails from the same clutch. Adult snails cease to grow and form a thickened lip around the aperture. However, the snails sometimes mate already shortly before the lip is formed. The life span of "H. pomatia" may reach 30 years in the wild. However, in the wild, they mostly live for just 5–9 years. The age of "H. pomatia" individuals can be estimated by counting growth interruptions on the shell and the number of layers deposited on the aperture margin lip. 
Data from other species are very limited. "Helix lucorum" in Greece reaches maturity at 3 years. The small species "H. ceratina" reaches maturity after 1.5–2 years and lives 4–5 years.
The length of the life cycle is dependent on environmental conditions. The time from hatching to first egg laying can be shortened in "H. pomatia" to just 12–13 months under optimal conditions in the captivity, mainly by skipping hibernation.
Ecology and behaviour.
Species of "Helix" live in a variety of habitats and under very different climatic regimes. Some species are exclusively found in open limestone rocky habitats ("H. secernenda"), while others tolerate acidic bedrock (e.g. "H. pelagonesica")