Mollusks




  • Mollusks are soft bodied animals which have an external and sometimes an internal shell
  • They belong to the phylum Mollusca
  • They are the second largest invertebrates and one of the oldest since they date back to the Cambrian period
  • They have been discovered to have a common ancestor in annelids about 550 million years ago and there are about 50,000 known species, which provides variety for mollusks.


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Forms and Functions


Mollusks basic structure includes a foot, shell, and a mantle. The foot is one of the most important for mollusks because they use it for pretty much everything. The foot can either be used for moving, which means it would be flat, or for burrowing which means its spade-shaped, or it could be in the form of tentacles that are used to catch prey. The mantle is a thin layer that covers the body of the mollusk and the outer shell is made from glands found in the mantle.
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All mollusks contain three body zones. These are the head-foot which contains sensory and motor organs, the visceral mass which contains the organs having to do with digestion, excretion, and reproduction, and the mantle. Another important part of the mollusk is the radula, which is more commonly known as the tongue. The radula is made up of chitin and it has different purposes depending on the animal. The radula in the herbivorous mollusks is used to scrape off algae from rocks and such. It is also used in combat for some mollusks.


Digestive Tract



The digestive tract is very similar to that of humans in that it has a mouth, anus, and stomach. Food is taken in by the cells and then is passed into the blood. The food that is undigested is compressed tightly into a sort of "package" and is passed to the anus which excretes it and then water currents carry it away. Nephridia is used for the excretion process in a mollusk. They are tube like structures inside the mollusk empty the waste into the mantle cavity.



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Nervous System


Mollusks are known to have a quite complex nervous system. The most complex of these nervous systems is found in the octopus. The octopus is considered one of the smartest invertebrate with high mental capacity. Sensory ability is also very high and mollusks (especially cephalopods) have large complex eyes that allow for easy movement. The largest eyes in the animal kingdom belong to the giant squid, which is a mollusk. They need these big eyes because since they live so deep down in the water where light doesn't reach it, prey is very hard to find but the giant squid has survived using its large eyes.
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Respiratory System


All mollusks except for cephalopods have an open circulatory system. This means that they collect their blood from the gills that was pumped through the heart and then it is distributed throughout the spaces between tissues and then it goes back to the heart and gills. The reason cephalopods do not have an open circulatory system but a closed one, is that out of all mollusks they use the most energy therefore they constantly need a huge amount of blood being pumped through their cells. Since this system allows for more blood to be pumped through the body of a mollusk more constantly and in more loads, then it has led to an increase in the body mass of mollusk. A perfect example is the giant squid who is known to grow to 18 meters long.
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Variety


The three types of mollusks are bivalves, cephalopods, and gastropods. Bivalves include clams, oysters, mussels, and scalops. They are called bivalves because, as their name suggests, they have two shells held together by a strong muscle. This muscle is used to open the shell and close it to make a type of propelling force to move the animal. Bivalves have no defined head and have no tongue either. They do however have a foot that is laterally compressed (for moving). Most bivalves have poorly developed sense organs except for a few exceptions. Their nervous system is based around three ganglia. Bivalves are dioecious and fertilization can be internal or external. Most bivalves are herbivorous and filter feeders, using cilia to bring in food particles. The cilia create currents in the water.
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Cephalopods are among the most intelligent invertebrates on earth. Some can even challenge vertebrates for intelligence. Currently, there are about 600 species of cephalopods. Throughout the years, some cephalopods have lost their outer shells. Some examples are the squid or octopus. The only thing protecting them is their mantle. Cephalopods move through the water in a dart like motion using a type of propelling system. Water is sucked into the mantle cavity and then fills up the gills, and then it is expelled through a tubed structure called a siphon. The muscle that contracts to expel the water regularly does it very slowly but in time of need such as when its being hunted or its hunting, it can quite suddenly contact the muscle very quickly and make a fast propulsion. Also, they have formed a type of defense mechanism against predators. When they are being chased, they can excrete a type of dark liquid that surrounds the enemy and clouds its vision, confusing it. Cephalopods have seperate sexes and fertilization occurs internally. They have complex mating rituals that normally include the male fighting for access to the female.
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Gastropods are the most abundant type of mollusks. These include animals such as snails or slugs. They have either a single shell or possibly no shell at all. They can be found in about every single habitat. They can live in salt water, fresh water, and on land.
They have a wide variety of diets depending on the animal. They can either be herbivores, omnivores, carnivores, scavengers, and some are even parasites. Snails living on land do not have gills anymore, but their mantle cavities used to contain gills inside them, so that are has blood vessels running through it. Since they move constantly they have a ganglionated nervous system. Some gastropods reproduce through external fertilization and they have separate sexes. Now though, most of them reproduce internally and there are more hermaphroditic cases. They generally follow a pattern of being males when they are young and as they mature and grow physically, they turn into females.
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