Anatomy

External anatomy

snail diagram

Land snails have a coiled shell to which their body is attached by a strong muscle. The snail can withdraw its body completely into the shell when it is resting or protecting itself from a predator. A snail cannot leave its shell any more easily than humans can leave their fingernails!

The shell stays with the snail for its life, and grows along with it. When a juvenile snail hatches from an egg, its shell has only one whorl, that is, one 360 degree rotation from the center. As the snail grows, the shell increases in size by growing around the first whorl – and around, and around, until the snail has reached its adult size. Different species of snails have different numbers of whorls.

The snail moves about by means of a large, muscular structure called a foot. Waves of muscular contractions and expansions enable the snail to move forward. The snail secretes mucus from its skin, which lubricates the surface upon which it crawls. With its muscular foot and marvelous mucus, the snail can easily glide over many surfaces, including glass, on which it can move vertically and even upside-down. A snail can even move over a sharp razor blade without getting cut, thanks to the protective and slimy mucus.

Land snails breathe air, just like we humans do. Air enters the snail’s lung through its pneumostome. Inside the lung, which is a region of tissue where gas exchange occurs, the oxygen is picked up by the blood, and carbon dioxide is emitted – again, just like human respiration.

The mantle is a fold of skin that surrounds the snail’s internal organs.

 

Internal anatomy

Under their shell, land snails have organs and systems just like humans do. The animal’s internal organs, which include a digestive gland, lung, heart and reproductive organs, are located within the mantle cavity, inside the shell of the animal. In slugs, the organs are just behind the head region.

Land Snail Sense organs

Land snails have three main sets of organs for sensing the world around them, as shown by this diagram.

Coppolino_snail_senses_labeled_sm_cTentacles: The snail’s head features two sets of tentacles: upper (posterior) and lower (anterior). The upper tentacles are longer and have eyes at their tips, in most land snails, as well as olfactory neurons for smell/taste. No one knows how well snails can see, but they definitely respond to light and are able to see where they are going as they move along, and their eyes do have a retina, lens, and optic nerve, just like our eyes. The lower tentacles are short and are used for smelling and tasting to help the snail find food and also avoid areas that could be potentially dangerous.

Nervous system: Land snails have various types of nerve cell endings, which arise from bipolar and multipolar cells in their epidermis, or skin. These enable the snail to feel a sense of touch and pain. When touched on its soft body, the snail can sense whether there is danger and will retract into its shell, if necessary.

Statocysts: Land snail statocysts are hollow balloons surrounded by cells with a ciliated (hairy) interior that are filled with a lymphatic fluid. Tiny calcium particles, called statoconia, float within. When the snail is upright, the statoconia float upward, touching the cilia and signaling to the snail that it is upright. When a snail is upside down, the statoconia float upward to what would ordinarily be the bottom of the statocyst, signaling to the snail that it needs to pull itself upright. The statocysts are located primarily inside the snail’s foot region. Although snails don’t have a real sense of hearing the way we do, it is believed that the cilia, those tiny hairlike projections, can sense sound vibrations, further giving the snail information about its surroundings.

Shell anatomy

The features of the snail shell have different names. Diagrams coming soon!

 

Shapes of snails

Land snails come in many different shapes. Learning to identify a snail begins with describing its overall shell shape. Diagrams for globose, beehive, bullimoid, and conical shells coming soon!

 

Illustrations ©2007-2015 Marla Coppolino