19 June 2010
The author of a fanciful but intriguing study of ancient man has analysed the rock-carvings at a Stone Age temple in Ireland, and deduces that the priests or shamans who built it had discovered one of the secrets of the cosmos, the underlying plan on which all growth and development are based.
The calculations are beyond me, but the idea is that all development can be reduced mathematically to a single pattern: the spiral, or helix. The spiral is seen in the shape of galaxies, in the flow of water, in the growth of plants following the sun, in the structure of DNA (the basic substance of genes), and, more abstrusely, in a thousand other aspects of the natural world.
Nowhere is this pattern more perfectly expressed than in the shell of the humble snail, such as the one I found this morning on the garden path. Certain species have a shell so regular, so much in accordance with the mathematical theory, that it could have been constructed for the sole purpose of giving it tangible form. But then everything in nature conforms to the rigorous laws of survival, and simplicity and beauty are mere by-products of this strict adherence to function.
The helical shell is a brilliant piece of design. Because of its shape, it can be continuously enlarged without the need for moulting. Insects must cast off their hard outer skeletons at regular intervals during growth, but the molluscs (the group of animals which includes the limpet, periwinkle, and snail), by inventing the shell, have secured permanent protection for themselves.
Originally all molluscs lived in water, and the majority of them are still to be found there, whether in the sea or in fresh water. In making the transition, those sorts that colonized the land met with a new set of problems.
The shell was the main asset that allowed them to make the change, protecting the animal from water loss. The shell is laid down in two layers. The inner one is made mostly of calcium carbonate, but the outer is hard, virtually impermeable, and horny. In unfavourable conditions, a snail can seal the mouth of its shell, either with a horny plate or with a temporary shield of dried mucus, and survive until conditions are more to its liking.
Every advantage, though, has its disadvantage, and the disadvantage for the snail is the weight of its portable home. While saving the animal from extremes of dryness and heat and from smaller predators, the shell slows it down and makes it easy prey for larger ones. A revolting and unmistakable sound made by badgers is heard when crunching snails; and of course song thrushes are well known for their habit of making an anvil – a favourite stone on which snails are bashed to bits. One such anvil I found recently was surrounded by the debris of a least a dozen shells.
Snails do not have a very appealing image, and still less do their relatives, the slugs (which, wisely or not, have dispensed partly or wholly with that cumbersome shell), but they are fascinating and admirable creatures for all that, and deserve a better response than the customary “uurgh” when found lurking behind a leaf or under a stone.
The colours of living snails are very subtle and beautiful, and, looked at dispassionately, so are those of slugs. But it is for their biology and behaviour that these animals are most worthy of study.
Both slugs and snails are most active by night, hiding out during the day in some damp place. With a few exceptions, they are vegetarians, eating especially plant material which has begun to decay. That is the reason they can be a pest in the garden: cultivated plants are often much softer and flabbier than their wild counterparts. The food is rasped to pieces between the upper jaw and rows of horny teeth on the tongue; these teeth grow continuously from behind. Snails restrict their wanderings to the surface of the soil or above, but slugs can dig into the ground as far as a metre, and are often the culprits when root vegetables are attacked.
As a student I once had to dissect a snail, which is an experience I wouldn’t wish on my worst enemy; and we will not dwell here longer than absolutely necessary. Nonetheless the internal organs, if you can sort them out, comprehensively fulfil all the functions that a snail could wish for.
Most complicated, perhaps, are the organs of reproduction. Snails are hermaphrodite, with a single gonad, which produces eggs for a short period each year and sperms for the rest of the time. As in most hermaphroditic animals, self-fertilization is rare, and the courtship of the snail is not without its own bizarre charm.
The eyes are tiny, usually on the tips of one of the two pairs of tentacles, and the vision is poor, so it is unlikely that the lovers’ eyes meet across a crowded shrubbery. Neither could they rush together to consummate the embrace; by the time they made contact they would both have forgotten what they were doing.
No: contact is probably made by smell, for the tentacles are equipped with sensitive olfactory cells. The two snails circle each other, frequently touching, and form in the process a platform of accumulated slime. Finally, the partners discharge at each other what are known as “love darts” – sharp spicules of calcium carbonate, variously shaped, but often winged in cross-section like the feathers of an arrow. The dart lodges in the tissues, where eventually it is absorbed.
It serves as a stimulus for completion of the act. The snails align their reproductive openings, and coupling begins. Sperms are exchanged in the form of packets; fertilization may not take place for up to a year after mating, but egg-laying usually follows fertilization within about a fortnight.
The eggs are laid in the soil. The baby snails resemble the adults in most respects but size, and, if they survive, can expect to reach maturity in about a year. The shell is incremented in stages, with material added to the leading edge, so that “growth rings” can be seen on a shell, rather as on a tree stump. When adulthood is attained, the shell usually stops growing and a thickened lip is formed at the mouth.
So ends the mathematical growth of the helix. I wonder if the brown and cream-banded snail I found on the garden path knows or cares much about the underlying forces of the cosmos.
Somehow, I doubt it.