Learn to be digested, and undesirable materials are

Learn about the animal phylum Mollusca and its most important body systems. Mollusca includes a wide range of common invertebrate species that can inhabit land, freshwater, and saltwater environments, making them an extremely diverse group of organisms.

What Are Mollusks?

Chances are, you’ve encountered plenty of members of the phylum Mollusca in your lifetime, but perhaps never realized they were designated as mollusks. It’s estimated that there are more than 100,000 mollusk species currently living on Earth (and well over 30,000 extinct species), many of which – like land snails and slugs – you can find living in your own backyard.

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You might even have eaten mollusks in the form of escargot, calamari, squid, scallops, clams, or oysters.The phylum Mollusca is one of the larger invertebrate animal phyla, and its members can be found on land, as well as in both freshwater and saltwater environments. Only the phylum Arthropoda contains more invertebrate organisms. Almost a quarter of all marine species are members of Mollusca.

All mollusks are soft bodied, meaning they lack the rigid internal skeletal structure that you and I have, and they share three common body parts: a mantle, a visceral mass, and a foot. As you might imagine, the foot helps provide the organism with a means of motion in the form of a mucus-covered body part that glides across surfaces. The visceral mass is where the animal’s major body systems (including the digestive, nervous, and circulatory systems) are found.

And lastly, the mantle is a layer of tissue that acts as a flexible, protective wall around the visceral mass and the animal’s other internal parts.While there may be variations in other parts, such as having a shell or sensory appendages, all mollusks have some form of mantle, visceral mass, and foot.

Digestive System

With its three main body parts, the overall structure of a mollusk is simple, but its digestive system is far more intricate. While there are slight variations depending on if the mollusk is a carnivore, omnivore or herbivore, the general setup of the mollusk’s digestive system is the same. Like humans, mollusks have a mouth, esophagus, stomach, and anus.Their mouth, also like humans, contains teeth as part of an oral structure called a radula.

But unlike human teeth, which come in different shapes for different purposes and in predetermined numbers, radula teeth can exist in different numbers, and they all have one sole duty – scraping. What gets scraped varies by the organism, but can range from plant material for land mollusks to the hard coralline algae found on coral reef structures for ocean mollusks. The radula is also supported by a unique structure called the odontophore, which is made from cartilage. Mollusks are the only organisms on Earth that have an odontophore.Once the radula scrape food into the mouth, the food encounters mucus (sort of like mollusk saliva), which coats it and makes it easier for cilia-like structures to help move the food into the stomach. The mucus remains attached to the food as it travels into the stomach, forming a mucus string. The mucus string remains attached through the stomach, and at the end of the stomach system, it’s wrapped around on itself prior to being excreted.

The mollusks stomach acts as a kind of sorting facility, where desirable nutrients are sent to be digested, and undesirable materials are sent to be excreted without being digested first. All mollusk wastes – both digested and undigested – are eventually excreted from the body by an anal structure that allows the waste to pass through the mantle and out of the body entirely.

Nervous System

While many mollusks don’t appear to have obvious sensory organs, most actually have a more than adequate sensory and nervous system. Not all mollusks contain a true brain, but all have either two or three pairs of nerve cords that are contained within their visceral mass.

One pair is for serving the foot region, another for the visceral region, and, in bivalves, a third is designated for the muscle that controls shell motion.You might already know that the human nervous system contains ganglia for regulating nervous function; well, mollusks have ganglia as well. The mollusk’s ganglia serve a range of functions depending on where they’re located. Perhaps the most important to the animal’s survival are the ones that process external stimuli from the eyes because these are the ones that can help the organism evade predators and find food.

The foot ganglia are also important because they help to regulate the overall motion of the animal. As you can probably imagine, the mollusks with the most sophisticated nervous systems – the Cephalopods (octopuses, squids, and chambered nautiluses) – are the ones with the most complex bodies, feeding habits, and interactions.

Circulatory System

Unlike humans, who have a closed circulatory system, mollusks have an open circulatory system in which oxygen and carbon dioxide are exchanged across gill filaments.

The gill filaments are located on the edge of the mantle and act as a barrier between the body’s blood and water or air outside of the body. Some mollusks can breath in only water or only air, whereas others (particularly coastal marine species) have adapted to be able to function in both air and water environments.Mollusks contain blood that is circulated around their body cavity in a wide variety of ways depending on the species, but the majority contain a heart-like muscle that pumps the blood throughout the body. The heart can contain multiple sets of atria and ventricles (again, it depends on the species), which function similarly to human atria and ventricles.The blood of mollusks contains a special chemical called hemocyanin, which helps carry oxygen throughout the bloodstream. Oxygen-rich blood will appear blue, and oxygen-poor blood will appear colorless because of the presence of copper compounds within the hemocyanin. Note that human blood derives its red color from iron compounds, which are lacking to non-existent in mollusk blood.

Lesson Summary

The phylum Mollusca is one of the largest and most diverse groups of invertebrates on Earth. They can exist in land, freshwater, and saltwater environments. All mollusks have three main body parts: a foot, a mantle, and a visceral mass. Their foot controls motion, their mantle acts as a protective barrier around their internal body parts, and the visceral mass contains important body systems like the stomach and nervous system.They have a well-defined digestive system that includes a radula for scraping food from surfaces.

The digestive system is similar to humans in that it contains a mouth, esophagus, stomach, and anus.Their nervous systems are more complex than they may seem and include nerve ganglia to help transmit messages around their bodies. Lastly, mollusks feature an open circulatory system, in which gases are exchanged across gill filaments, and their blood contains a special, copper-rich compound called hemocyanin, which transmits oxygen through their system and gives their blood a blue appearance. Overall, the members of phylum Mollusca are far more sophisticated and complex than they may outwardly appear.


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