Phylum Mollusca-Helix, Biology tutorial

Introduction:

The giant African snails, Achatina achatina and A. fulica, are gastropods; they belong to same subclass and are anatomically very similar to Helix pomatia. The pulmonates, of which Helix is one, are mainly characterized by conversion of mantle cavity into air-breathing lung with contractile opening, pneumostome.

Helix:

Genus Helix, a member of subclass Pulmonata, is gastropod that illustrates molluscan adaptations to terrestrial mode of life. Helix pomatia, Roman snail, is largest British pulmonate and contains long history of being eaten by humans. It is thought that Romans introduced H. pomatia to Britain, therefore its common name. Helix pomatia lives in open woodland, banks, quarries and hedges on calcareous ground, but not generally on cultivated land like gardens or parks. Helix pomatia buries itself just below the surface of soil, while H. aspersa congregates in groups in hidden places or may also bury itself. In hibernation shell aperture is sealed with the operculum that is membraneous diaphragm of hardened mucus impregnated with calcium carbonate and small quantities of silica, phosphates and iron.

The shell:

Like in all shell-bearing gastropods, most conspicuous external characteristic is shell that is spirally coiled. Structure, can be described by as elongated, hollow cone, wound round central hollow axis, columella. Columella opens at small ventral pore or umbilicus and sutures between coils are closely fused. Shell slowly grows longer and wider in the increasing spiral shape, to better accommodate growing animal inside. Animal also thickens shell as it grows, so that shell remains proportionately strong for its size. Helix is known to store excess calcium carbonate in cells in digestive gland. When reared on the diet which lacks calcium, resulting shell is thin and transparent or it may fail to give complete coverage for visceral mass/hump.

The body:

When snail is resting head and foot are pulled in large chamber created by lowest whorl of shell, by contraction of columellar muscle. This muscle creates on columella, runs down side of visceral mass, and splits in bundles extending to head and anterior and posterior foot. Foot is very conspicuous, broad and very muscular with flat undersurface; head bears 2 pairs of tentacles and slit-like mouth. Rest of soft parts of body form the visceral hump/mass that is always surrounded by shell. Lower margin is thickened and fused with edge of mantle to form the collar. These last are secreted by mantle.

Locomotion:

Locomotion in Helix can be compared with that of gliding movements of large freeliving turbellarians (flatworms). Flattened ventral surface (sole) of foot is pulled in series of minute ridges by waves of contraction of pedal musculature. Musculature is complex web (including dorsoventral, longitudinal, transverse and oblique fibres) whose contraction is handled through the nerve net by pedal ganglion. The edges of ridges are directed backwards and waves of contraction pass from posterior towards anterior end.

For gliding movement of this type it is essential for surface to be lubricated; essential lubrication is given by mucus produced by epidermal glands in sole and by anterior glandular mass the pedal gland.

Digestive system and Feeding:

After the snail has taken its food, rasping it in tiny little bits with its radula, food disappears in snail's gullet to be digested. First steps of digestion already occur in snail's mouth. Here saliva, generated by two large salivary glands both sides of crop, digest carbohydrates, like starch and sugar. Proteins and celluloses, although, are not digested by salivary enzymes. Saliva also change food in pulp which can easier be transported in gullet.

At its rear and stomach or crop is blind sack, in which food pulp is dammed. Here major part of digestion occurs. Digestive fluids are generated by main digestive gland or hepatopancreas.

In contradiction of saliva, digestive fluids from hepatopancreas can digest not only carbohydrates, but also proteins and lipids. Roman snail is so given with array of enzymes, that also can digest substances indigestible for humans.

From stomach blind sack, digestive tract is continued by intestine. It starts with the ciliated groove in stomach, by which indigestible parts of food are transported towards exit. There is final resorption of simple sugar molecules in intestine as well. Snail's intestine lastly opens outside by anus that is situated near respiratory opening in apertural mantle fold. Dark thread of faeces can frequently be seen, when the crawling snail just leaves it behind.

The Main Digestive Gland:

Major digestive gland of hepatopancreas, is largest gland in Roman snail's organism. Large, dark brown organ, it makes largest part of visceral sac inside shell. Although organ is frequently referred to as liver term hepatopancreas or main digestive gland is more accurate. Snail's main digestive gland generates digestive fluids and stores nutrients. It also carries out digestion as such and resorption that means it extracts nutrients from digested food. Digestion changes food pulp in diluted juice rich in nutrients, which flows from stomach in lobes (follicles) of digestive gland. Enzyme cells there generate digestive fluids, while liver cells extract nutrients from food juice.

Gaseous exchange:

Major feature of pulmonates is conversion of mantle cavity in air-breathing lung. Mantle cavity is altered to form lung by fusion of mantle edge to back and neck in front of visceral hump. Lung is roofed by mantle that is widely supplied with blood by pulmonary vein. Floor of lung is formed by head and foot which, in their relaxed position, arch up in lung but can be flattened by contraction of intrinsic longitudinal muscles. Gaseous exchange occurs over surface of mantle, with air being drawn in mantle cavity through pneumostome by alternate arching and flattening of lung floor. Pneumostome can be closed by the valve, to decrease water loss.

Circulatory system:

In pulmonates like Helix, heart comprises of single auricle and ventricle lying within pericardial cavity. Blood from both general body surface and pulmonary vein is received by auricle and pumped in ventricle. Single aorta, arising from ventricle, divides to form cephalic artery that supplies anterior part of body and foot (as pedal artery), and visceral artery that supplies visceral hump. Arteries branch and enter a series of irregular cephalic, pedal or visceral haemocoelic spaces or sinuses in which the organs lie bathed in blood. Blood has copper-based respiratory pigment, haemocyanin that provides it distinct green colour.

Excretion and Osmoregulation:

Helix contains single kidney, representing that of post-torsion left-hand side. It is a sac of coelomic origin linked to pericardial cavity by renocardial canal and drained by kidney duct, or ureter that runs along rectum to slit-like opening at right edge of pneumostome, close to anus. Both renocardial canal and ureter are ciliated; in former cilia beat towards kidney creating the flow of fluid, though in Helix canal is tiny and flow of fluid small. Kidney is yellow and contains folded glandular walls which secrete uric acid that can be discharged in solid form, thus conserving water. Uric acid can be stored in kidney over long period; this is significant factor in survival through periods of hibernation. Greater part of fluid that enters kidney through renocardial canal is reabsorbed.

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