This is a roman snail. Everyone recognizes a snail by its twisted shell, but that is not the only interesting thing about a snail. What a snail looks like on the inside is very special. The organs look different, but you will recognize most of them because they usually do the same thing as our organs.
A snail's shell can have different colors and stripes. It protects a snail from drying out. Part of the snail always remains in the shell. The body of a snail is only connected to the house in one place: only the muscle in the foot, which ensures that the snail can retract into the shell, is attached to it.
The body of a snail is soft. A snail can move because of a large muscular "foot". That foot slides over a surface via a slime trail. That mucus is made by mucus glands in the foot. A snail's head has two pairs of tentacles with the top pair having eyes at the end.
Most of a snail's organs are located in the shell, inside the "mantle". These are the digestive gland, the lung, the heart, the stomach, and most parts of the reproductive system. The “foot” also contains organs: the crop, the salivary glands, the penis and vagina, the mucous glands, and the dart sac.
The digestive system of a snail is simple. Most organs are clearly recognizable.
The buccal mass of a snail is its mouth cavity. This large, ovoid mass of muscle contains the pharynx and radula. The radula are the teeth of a snail.
Large sac located beyond the esophagus, where food is held before being digested in the stomach.
The salivary glands, just like in humans, produce saliva for the snail's oral cavity. And just like in humans, the saliva provides enzymes that already digest the food a little.
The stomach is a widened part of the digestive tract.
This gland consists of two parts and produces digestive juices.
The poop hole
Snails have a very simple heart consisting of two chambers: an atrium and a ventricle. Between the two is a narrow passage where a valve prevents the blood from being pumped in the wrong direction. There is a sac around the heart, the pericardium (just like in humans, but not completely. Look at the kidney for this). A snail's heart rate depends on their body temperature and how active a snail is. Sometimes a snail reaches 70-80 heartbeats per minute, but when a snail hibernates it goes back to about 5 beats per minute.
To breathe, a snail lowers the diaphragm, the bottom of the lung, just like humans do. Oxygen enters the blood through the veins in the wall of the lung. And carbon dioxide goes out in the same way. A snail can control this process even more by closing or opening the entrance to the lung.
Like most other mollusks, a snail's blood circulation is open. The aorta branches into many smaller veins that supply blood to different parts of the body. From the smallest veins, the capillaries, the blood flows into the body cavity. Capillary veins carry the blood back to the lung where the blood again absorbs oxygen and releases carbon dioxide.
The kidney of a snail is called a nephridium and has a similar function to our kidneys. However, the blood is filtered from the heart in the pericardium around the heart and from there goes as a kind of "pre-urine" to the kidney via the "renopericardial canal". This urine contains not only waste products but also nutrients. The surface of the kidney is very "folded". Think of the inside of our intestines. This has greatly increased the surface area. The kidney filters as many nutrients as possible from the fore urine. What remains is the actual urine. It leaves the snail through the ureter, which ends next to the anus.
The reproductive system of a snail is very special: a snail is both male and female. To ensure that a snail does not fertilize its own eggs, the reproductive system is very complicated. The names are also complicated, but the tabs explain what each organ in the reproductive system does. Because the reproductive system is so complex, read the tabs several times in different orders.
This is where all the exits come together. That of the penis, vagina, and the dart sac.
The vagina is where the spermatophore of another snail ends up to fertilize that snail's eggs.
The spermatophore containing the sperm cells is introduced into the vagina of another snail with the help of the penis.
Because snails are of course both male and female, snails have figured out the opportunity of one's own sperm cells to reach the eggs of another snail: the love dart. The love dart, which is made by the mucous glands, contains hormones that cause the copulatrix bursa to open so that more sperm cells can escape. The hormones also ensure that the oviduct undergoes peristaltic movements that bring the sperm cells toward the spermoviduct.
The mucus glands not only produce hormones for the love dart but also provide a mucus layer around the eggs to protect against fungi and other infections.
Epiphallus and flagellum
The spermatophore has already passed above. The spermatophore is made at the border of the vas deferens (sperm duct) and the epiphallus. Gland cells in the epiphallus produce a layer on the inside of the epiphallus. On the side of the vas deferens the cavity remains open, otherwise it is closed. The spermatophore has a thread-like tail. It is formed in the flagellum. This is a whip-shaped organ at the end of the epiphallus. The spermatophore of Helix pomatia is approximately 11 cm long including the tail!
The ovotestis, as the name suggests, produces both eggs and sperm.
Just like in humans, sperm cells are continuously produced. The sperm cells are stored in seminal vesicles. As soon as the sperm cells are needed, they travel via the hermaphrodite duct to the spermoviduct.
Egg cells are only transported from the ovotestis to the fertilization sac when they are needed. There, fertilization takes place by sperm cells from the sperm sac. When they leave the fertilization sac, the large protein gland ensures that the now-fertilized eggs are provided with a nutritious protein layer. Despite the name of the gland, this nutritious layer consists not only of protein, but also of glycolipids (lipid, so a fat particle, with a sugar unit attached to it. These ensure that the snail embryo can develop properly) and other nutrients.
After mating of two snails, the bursa copulatrix pulls in the spermatophore with peristaltic movements and begins to enzymatically digest both the spermatophore and the sperm. However, with a length of almost 10 cm, the tail of the spermatophore is considerably longer than the stem of the bursa, so that some sperm cells leaving the spermatophore are almost at the entrance to the oviduct and out of danger. Although the sperm cells can leave the spermatophore through a posterior opening, usually only 1‰ of the sperm cells manage to escape.
Within the spermoviduct, the vas deferens lie as an open groove attached to the fallopian tube. To prevent self-fertilization, the connection is closed during mating, so that the sperm cells can only follow the vas deferens. There is also a gland attached to the vas deferens that resembles the prostate. It produces a secretion that provides the passing sperm cells with energy and nutrients. It also immobilizes them, so from here the sperm cells are transported by peristaltic movements. Shellless eggs are transported to the oviduct portion of the spermoviduct, where they are laid out in a line like pearls on a chain. This part of the spermoviduct is also called the uterus because it contains gland cells that provide the eggs with different coating layers that form an eggshell.
The few remaining sperm cells still have a long way to go towards the end of the spermoviduct. Here they enter the fertilization sac. A specially detached blind side pouch serves as a sperm sac or spermatheca. This is where all the sperm cells from all the copulations that a snail has managed to arrange are stored. For fertilization, they only need to leave the sperm sac and enter the adjacent fertilization sac.
Snails don't have brains like us. The brain is divided into ganglia, nerve nodes, with different functions.
The two cerebral ganglia are closest to what you might call the “brain” of a snail. Each cerebral ganglion consists of three separate parts, similar to a forebrain, a midbrain, and a hindbrain. From the forebrain nerves reach the tentacles, the optic nerve and the olfactory nerve, but also to the penis, and from the hindbrain there are nerves to the lips and the vestibular organ. After all, the middle brain is the computing center of all a snail's cranial nerves. It is also connected by connections to the other ganglia.
The pedal ganglia form the lower part of the nerve ring. Nerves from here mainly connect to muscles in the foot.
The 'visceral ganglion' consists of several ganglia:
From here nerves go to the stomach and the front parts of the intestine, and to the salivary glands.
The Anatomy of Helix pomatia
This app shows anatomy of the edible snail Helix pomatia. click on the underlying tabs to see an explanation per tissue.
© 2023. Mieke Roth