Today, I was outside my house when I saw a snail, and I decided to understand the anatomy of this very small and slow creature. It is important to know that this small creature is responsible (directly or indirectly) for the transmission of diseases to humans. According to the World Health Organization (WHO), Diseases such as Schistosomiasis, which is caused by Blood Flukes (trematode worm), is released by freshwater snails which kills about 200,000 people yearly. Other snail-borne parasitic diseases includes, angiostrongyliasis, clonorchiasis, fasciolopsiasis, opisthorchiasis paragonimiasis, amongst others which causes diseases in humans. Today, I am not discussing the diseases caused by snails, I want to look into snails anatomy.
Snails are of the phylum Mollusca, which is the second-largest phylum of invertebrate animals, and the largest marine phylum. Organisms in this phylum are squid, octopus, nautiloids, slugs and so on. Snails are usually classed as Gastropoda (stomach footed), but this class includes so many orders, and most of the land snails fall under the order Stylommatophora (with over 20,000 species).
The Internal Anatomy of Snail
A lot of us know snails with its shell and its body, but we need to know that beneath the snail shell, are organs and systems just like humans. You will be so surprised at how complex the snail can be.
Internally, I will start with the Radula which is the main feeding organ for mollusks (it is the same for snails). It helps to bring food into the mouth of the snail and grinding (just as you have your teeth for grinding). The radulla is supported by the cartilage Odontophore, which helps extend and retract the radula. In the Pharynx, is the salivary glands The salivary gland of snails could contain a range of 5 to 17 types of cells, ranging from monocytes, granular cells, Vasculated cells and many more. In the cone of the shell is also a venom gland. The saliva and venom are used to digest the food first hand. The saliva helps to digest starch, cellulose, and protein, converting to pulp for easier digestion and transportation into the snail's gullet. In the stomach of the snail, the digestion continues and the hepatopancreas. During the process of digestion, lime is gained from the food. This lime is then moved to the bloodstream, which is then transported to the shell-building cells. The food is then transported to the intestine from the blind sack, where digestion continues. The indigestible food is transported to be excreted from the intestine to the anus which is located close to the apertural mantle fold.
Snails have an open circulatory system with two heart chambers. The chambers are 1 atrium and one ventricle. While most snails are two chambered heart, some are 3 chambered heart (2 atria and 1 ventricle). Snails also have blood vessels such as the Pulmonary vein, connecting the heart to the lungs, the artery which circulates heamolymph to the organs. Heamolymph in its oxidised state usually appear pale blue. Some snails have hemoglobin, in the case of Planorbis (Ramshorn). The kidney in snails is important for excretion of indidestible food. While in water snails, excretion is much liquid but in terrestrial snails, water is absorbed from the waste. The kidney of snails is made up of interior walls called Septae. The blood in snails transport nutrients and blood to the snail body, and serves as a retractor muscle for the snail tentacles and the snail body. The snail body as well as genital (penis) is pushed out and withdrawn by blood pressure.
Respiration in pulmonate snails is done by the Pallial cavity which serves as a simple lung. In the Pillial Cavity, a double comb structure gills called ctenidia which helps in the gas exchange. Snails just like other animals breathes in oxygen and breathes out carbon(iv)oxide.
In reprodoction, terrestial pulmonate snails under the order Stylommatophora, and Opisthobranchia are hermaphrodite (having both genital apparatus in one). Depending on the type of snails, their mating process differs. In Leopard Slug, mating is done while hanging freely on its slime thread. Some other snails, such as the Roman snails, mate on the ground. After fertilization of the egg, oviposition (expulsion of the egg from the oviduct after fertilization) occurs. When the eggs are hatched, the young snails come out with all organs and shell. In some marine snails, reproduction process could involve larval stages. The first larvae stage being Trochopora, then the Veliger larvae. The Larvae float freely in the water.
Conclusion
The thought that snails do not have organs and system is completely wrong. The systems in snails go from circulatory, respiratory, digestive, reproductive, and nervous system. Snails do not have brains, in snails, the cell bodies (pericarya) in the nerve cells are present in the nerve knots (ganglia) which are available in different parts of the body.
https://www.who.int/news-room/fact-sheets/detail/schistosomiasis
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5890347/
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/mollusc
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/gastropoda
https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/panpulmonata
https://www.sciencedirect.com/topics/immunology-and-microbiology/stylommatophora
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/radula
https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/odontophore
https://www.sciencedirect.com/science/article/pii/S1687428518300529
https://pubmed.ncbi.nlm.nih.gov/15270238/
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/hepatopancreas
http://www.molluscs.at/gastropoda/terrestrial.html
https://www.scienceabc.com/nature/animals/how-do-snails-and-other-molluscs-create-their-shells.html
https://the-circulatory-system1.weebly.com/phylum-mollusca.html
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/hemolymph
http://www.molluscs.at/gastropoda/index.html?/gastropoda/morphology/respiration.html
https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/trochophore