4 Things About Land Snails

4 Things About Land Snails

Open Circulatory System

Snails are molluscs, possessing open circulatory systems, where blood isn't enclosed continuously in vessels, unlike in vertebrates. The heart pumps blood through arteries, which then end as they lead into big cavities in open spaces named haemocoels. Haemocoel cavities contain fluid called haemolymph - the previously mentioned blood, except not being 'true', as an open system entails mixing with substances like interstitial fluid; located between tissues.

Said cavities surround organs and tissues, where the blood circulates through and bathes them via direct contact - for greater temperature control - along with diffusion for cellular oxygen uptake and respiration. Blood then diffuses back into the enclosed part of the circulatory system through the heart ostia (pairs of lateral openings), as the system has no veins.

An open system implies less directional efficiency than closed, so substances are delivered around the body slower - amplified further by being cold-blooded since it's also generally slower than warm. Their low metabolic rate means snails can survive longer without nutrients, partly because they take longer to process.

Shells and Water Retention

Snails are born with shells and grow alongside them, like their own portable house, which they rarely ever leave. They carry their home on their back and can withdraw when in perceived danger.

-snails move in little wave motions on a single muscular 'foot', consisting of mucous to reduce friction, plus prevent abrasion and razor-sharp cut injuries; they can also save energy and get a speed boost on others' trails (travelling at 1 metre per hour otherwise)-

The shell is attached to the body via the 'mantle' - the muscular lining inside the shell that contains and protects a group of specific organs known as the 'pallial complex'. For further insulation, snail shells are strong, potentially resisting ~10x body weight above them - plus any damage can get repaired (unless destroyed entirely).

In extreme heat and dryness, structural adaptations reduce water loss. For example, snails have evolved thicker shell volumes and a denser epiphragm (a 'film' of dried mucous reinforced with calcium carbonate that is used temporarily during inactivity) to seal the shell hole, increasing the time an individual can afford to be sedentary. If one sleeps, the duration can range from just a few hours or up to a few years.

The aforementioned inactivity is a behavioural adaptation. Snails can reside in damp underground burrows, sheltering from harsh conditions and sleeping to conserve moisture, avoiding shrivelling/drying up. Any activity typically occurs at night, early morning, or in cloudy and/or rainy, wetter conditions.

Breathing

All terrestrial snails breathe air, where the mantle expands and compresses the lung muscles. In particular - lots of land snails have a 'pneumostome' - a little breathing pore on the mantle's side, which gets opened/closed voluntarily. When not actively breathing, the pore is usually closed to retain moisture.

Within the pore, oxygen in the air gets absorbed across the skin and into the body; while expelling carbon dioxide waste products simultaneously, increasing gas exchange adeptness. Food waste is also produced in; and expelled from the pore (the pallial complex includes the anus).

Tentacles and Radula

Snails have no ears or ear canals and cannot hear; instead, they have two pairs of tentacles, an upper, longer pair containing the eyes and the lower, shorter ones holding the olfactory system. They can retract the eye tentacles for protection and control over light exposure, as well as; quiver the nasal tentacles to maximise exposure to odour particles, or retract them as a sort of olfactory 'reset'.

Smell is likely their primary sense, indicated by their irregularly undersized chemoreceptors (presumably to fit more cells in for heightened sensitivity) that equate to a substantial abundance of nerve cells relative to the whole nervous system. Their smell mostly gets used for general direction, object identification, foraging and courtship.

Meanwhile, their mouth is apparently the size of a pin, except containing linings for thousands of microscopic 'teeth' projections on a ribbon-like tongue - sometimes ~20,000 of them, the most of any animal.

Altogether they form the 'radula', a prevalent feeding structure in molluscs. The radula can scrape and scratch for the most impactful force exertion to break food down. Snails also utilise it for a more tactile sensation when identifying objects - a human's hand, for instance.

Snail teeth robustness is similar to wood, but some foods can still be denser. However, their pointy cusps and sheer quantity transmit highly concentrated pressure to scrape away or even pierce the material.