Snails (Gastropoda)

Organs and Organ Systems


Schematic overview of a snail's anatomy:
1: Shell; 2: Digestion gland; 3: Lung; 4: Anus; 5: Respiratory hole; 6: Eye; 7: Tentacle; 8: Cerebral
ganglion; 9: Salivary duct; 10: Mouth; 11: Oesophagus; 12:Salivary gland; 13: Genital pore; 14: Pe-
nis; 15: Vagina; 16: Mucus glands; 17: Oviduct; 18: Dart sac; 19: Foot; 20: Stomach; 21: Kidney;
22: Mantle; 23: Heart; 24: Spermioduct (Vas deferens). Source: Wikipedia.

Of all molluscs, the snails have adapted to the most habitats, which has led to their amounting to the by far highest number of mollusc species. Adaptation to so many different habitats only way possibly by optimally adapting organs and organ systems. Alone from external appearance, two gastropod species may sometimes be hard to recognise as two of the same class, Gastropoda.

Respiration


Different in every regard, but nevertheless both are gastropods.
Left: Sea angel (Clione limacina), right: Roman snail (Helix pomatia).
Pictures: Kevin Raskoff (Source), left, and Cornelia Kothmayer (right).
 

Respiration is one of the characters very different between snail groups. Basically there are pulmonate snails, whose pallial cavity is formed to a simple lung to breathe oxygen from the air. Among those there are most of the terrestrial snails, as well as pulmonate snails living in fresh water.

 
The marine nudibranch Flabellina exoptata (Aeolidia) from
Malaysia. Picture: Choh Wah Ye (Source).

Other gastropod species have developed the ability to breathe air later. All marine gastropods, as well as some fresh water gastropods, breathe with gills. Snails originally possess the so-called comb-gills (ctenidia), which are also located in the pallial cavity. An exception are the marine nudibranchs (Nudibranchia), whose original gills have been reduced, so the nudibranchs breathe using newly developed feathery dorsal appendages called cerata.

Respiration and circulation of the Roman snail.

Circulation and Excretion

A snail's circulation basically is open. That means that snails may have some important blood vessels, such as the pulmonary vein, leading from the lung to the heart, and the main artery or aorta, but the blood circulates freely between the organs. There it mixes with the lymphatic fluid, resulting in the so-called haemolymph. A snail's heart has two chambers, one ventricle and one atrium. It is located in the heart bag, the so-called pericardium.

The heart bag is also important to the snail's excretion, meaning the disposal of indigestible material usually rich in nitrogen. In the most primordial gastropod species, filtration takes place through the heart bag's wall, the excretion taking place afterwards via an efferent channel, the urethra. Snails' excretion organs originally were derived from the segmented worms' (Annelida) metanephridia.

While water snails excrete a very much diluted primary urine, terrestrial pulmonate snails have developed the ability to resorb most of the water. In terrestrial snails, excretion takes place in a kidney, whose interior surface has been increased by many interior walls called septae. Through their walls filtration of blood flowing through takes place. Terrestrial snails usually excrete urea, containing almost no water.

Nutrition


A pond snail (Lymnaea stagnalis) grazing the algae on the
water surface. Note the visible radula! [RN]
 

As different as the methods of nutrition may be among snails (there are herbivores, omnivores and carnivores), it can nonetheless be retraced to common organs. To feed, snails use an organ, that is present in most molluscs and unique in the animal kingdom: The radula or a rasp tongue. Basically, it consists of an elastic band running over a gristle core and armed with a large number of chitin teeth. To feed, this rasping band is used like the transportation band of a bucket excavator, food particles rasped of are transported back into the gullet. The bow-shaped jaw is used to cut off food particles.

  Weinbergschnecke knabbert an einem Blatt.
A Roman snail eating a leaf of green food. It pulls the leaf into
its mouth using the radula and then cuts it off with the jaw.
[RN]

Similar to other animals' teeth, the radula as well is adapted strongly to the respective methods of nutrition. While plant-eating species, such as the Roman snail, have broad and blunt radula teeth, predatory snails usually have dagger or lance shaped radula teeth, enabling them to hold the prey and also to rip flesh from it (fittingly, a predatory snail from the American Northwest is called the Robust lance tooth - Haplotrema vancouverense).

Maximally adapted to the method of hunting and feeding is the radula of cone shells (Conidae): While elsewhere the radula carries thousands of teeth, cone shells only have one at a time, formed like a harpoon and used to inject the prey with venom. In marine gastropods, the radula is also often used for systematic purposes, which is why the difference is made between beam-tongue gastropods (Docoglossa, among them the limpets - Patellidae) or the venom-tooth gastropods (Toxoglossa, as expected, among others the cone shells - Conidae).

Radula Types.

Two salivary gland open into the gullet, which are used to digest food for the first time. In cone shells, they have been transformed into venom glands. A snail's stomach is a simple blind sac, in which the digestion by saliva continues. The main part of digestion takes place in the main digestive gland, a specialised gland taking most of the place in the visceral sac. It is also called a hepatopancreas, being both liver and pancreas. While usually the liver only produces digestive fluids and stores nutrients, in a snail's hepatopancreas also digestion takes place. Also, lime is gained from the food, later transported via the blood stream to the shell-building cells of the mantle.

Nutrition of snails.
Nutrition of the Roman snail.

Nervous system

 
Prof. Eric Kandel with a sea hare (Source).

The nervous system of gastropods is located at the ventral side and can be derived from the segmented worms' (Annelida) rope-ladder nervous system. While in very primordial gastropods, nerve knots (ganglia) are still placed in different parts of the body (the foot ganglion, the visceral ganglion, the pleural ganglion etc.), the nervous system of more advanced gastropod species, especially of terrestrial pulmonate snails, is very much centralised, all ganglia molten to form one common ring around the gullet, called the buccal mass.

Especially more advanced gastropod species are capable of rather astonishing neural feats - the marine slug Aplysia, for example, was used to research learning behaviour and conditioning.

Nervous system of snails.

Reproduction


Mating of the leopard slug (Limax maximus). Pictures: T. Hiddessen. (Wikipedia).
 

While basically snails have separate sexes, terrestrial pulmonate snails (Stylommatophora) are hermaphrodites, both male and female. So are some fresh water snails and the marine opisthobranchs (Opisthobranchia), altogether by far the largest part of all snails.

The hermaphroditic snails posses male and female, as well as hermaphroditic organs in one common genital apparatus.

Mating can be quite an interesting view in terrestrial snails. So for example leopard slugs (Limax maximus) mate hanging freely in mid air from a slime thread. In contrary to them, Roman snails mate on the ground, but their mating procedure may take several hours. Also, during the process, a love dart may be applied. Among the hermaphroditic Roman snail, copulation takes place afterwards simultaneously and vice-versa.

In terrestrial snails, the embryonic development of the young then takes place in the egg after fertilisation and oviposition. From the egg, complete young snails hatch, which have to grow, but which also have a complete set of organs and a shell.

On the other hand, some water snails, but especially the marine gastropods, reproduce via several larval stages (Trochophora and Veliger larvae), which swim freely or float in the water as part of the plankton.

Reproduction of snails.
Reproduction of the Roman snail.