Phylum Mollusca: Digestive, Nervous & Circulatory Systems

Phylum Mollusca: Digestive, Nervous & Circulatory Systems
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  • 0:02 What Are Mollusks?
  • 1:42 Digestive System
  • 3:35 Nervous system
  • 4:44 Circulatory System
  • 6:00 Lesson Summary
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Lesson Transcript
Instructor: Heather Pier

Heather has taught high school and college science courses, and has a master's degree in geography-climatology.

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. 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.

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