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| Sunday, November 9, 2003 |
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| Sunday, November 9, 2003 |
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NATURE
SNAILS have a soft body, which is enclosed in a cozy but hard outer covering. Though the strong shell provides them valuable protection against predators but, like all other armoured animals, it gives them a certain disadvantage. For instance, they are always less mobile, less flexible, and often suffer its consequences. However, they are less cunning, agile and exploratory. They are like rich people who usually remain insulated from the real hazards of life. The most interesting aspect of these lowly animals is their mating ritual. Like many lower forms of life, snails too are hermaphrodites, which means each sexual partner carries both male and female sex organs; nonetheless for sexual reproduction to take place a mate must be found, and then a sort of cross-fertilisation takes place with the sperm from each snail fertilising the ova of the other. Their mating is a sort of double marriage. Before indulging in the sexual act even the snails perform an elaborate courtship ritual, and this creates the problem of who ‘leads’, since both are male and female. When two reproductively mature snails have found each other, they circle around, coming closer and closer and when they are at a reasonable distance, both use their muscles to fire a ‘love’ dart at each other, which is carried in a small internal sac peculiar to each species. |
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In some species these love-weapons, which are made of the same calcareous substance as ordinary snail-shells, are large and painful; in others they are smaller and milder in their impact. For each species, the pain has to be felt at just the right level for it to be sexually stimulating. In this way there can be no error regarding the identity of the sexual partner. From both sides this ‘cupid’ dart travels with such a force that it penetrates the body wall and becomes embedded in the tissues of the internal organs of the other snail. This drastic stimulation is followed by normal copulation with mutual exchange of sperms. Snails are members of a group of molluscs that include about 77,000 species of slugs, limpets and whelks and they themselves have about 50,000 known species. Many of these gastropods (animals that use the underside of their abdomen to move about) live in the sea, though there are freshwater and land forms too. All snails live in a wet or humid habitat, but the desert snail (sphincterocheila) is an exception that lives in semi-desert conditions where water is very scarce and available only at infrequent and unpredictable intervals. In such conditions, the snail’s lethal limit for temperature, which is around 55 degrees C, is often exceeded with surface temperature frequently touching 65 degrees C. Despite all these odds the creature survives in surprising numbers, owing to its adaptations, which include a thick, white shell that not only reflects the sun’s energy but also prevents loss of water from the body. The shell has a small aperture too that can speed up water loss, but the provision of a robust mucous membrane, which can be thrown across the aperture to stop fluid loss, takes care of the possible danger. The shell’s shape is such that it helps not only in reflecting up to 95 per cent of the sun’s rays, but also prevents it from heating up. Due to its peculiar construction it touches the hot ground only at a few points. Apart from this, if the creature is not active it retracts into the shell leaving an insulating air layer. The cumulative effect of the entire strategic adaptations is that the temperature of the animal’s tissue remains around 50 degrees C even when the ground is at 65 degrees C. Lying totally insulated against the outside heat in its shell this gastropod can survive more than three years’ dehydration, rapidly becoming active when rain occurs. Most snails possess a
single, often coiled shell. They have a distinct head, with eyes and
tentacles. Many eat plant matter but some prey on other animals and feed
by scraping food material with radula – horny tongue covered with tiny
teeth. While feeding they rasp away at the food like a file or a strip
of sandpaper. The front end of the radula is always wearing away, but it
keeps on growing from the base, rather like fingernails, and pushing new
rows of teeth forward. The base of the body consists of a flat, muscular
foot. When the animal retires into its shell, the muscular foot is the
last part to be drawn in, so it is left blocking the shell opening.
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