The Phylum Mollusca is made up of over 150,000 diverse species appearing very different but sharing certain basic characteristics.


All mollusks have a muscular foot used for locomotion, as well as a mantle, an outgrowth that covers the animal. Many mollusks

have an external calcium carbonate shell, hanging over the mantle, that is actually produced by the mantle. The mantle is also used

in respiration, waste disposal, and sensory reception. It often creates a mantle cavity, where the gill is housed. The gill of a mollusk

extracts oxygen from the water and disposes of waste. All species of the phylum Mollusca have a complete digestive tract, spanning

from the mouth to the anus. Many also have a radula, a unique organ composed mostly of chitin, in the mouth. The radula allows the

animal to scrape food from surfaces, especially the ocean floor, by sliding back and forth.


Mollusks have a coelom, which is develops from solid cell masses. This makes all species in this phylum protosomes. All organs

are suspended in mesentary tissue within this mesodermic coelom, between the outer covering (ectoderm) and the digestive tube

(endoderm) of the animal. Interestingly, the phylum Mollusca is one of only two major coelomate phyla that do not have body

segmentation (the other being Echinodermata, as well as other lophophorate phyla). There are seven main classes of mollusks.



Class Polyplacophora

Class Polyplacophora, the chitons, have an external shell with eight overlapping plates. They normally live on rocks and scrape

algae with their radula. Because the mantle secretes these eight plates, chitons can bend to fit their home.



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Class Aplacophora

Class Aplacophora are mollusks without shells. They usually resemble worms, and are found in deep water, often more than 3,000

meters. There are only about 300 species in this class, and besides lacking shells, they are all marine and generally very small.


Class Monoplascophora

Class Monoplascophora, meaning having one shell, are a very small group that has been known of for a short time. Originally

thought to exist only in the fossil record, they were rediscovered in 1952, and there are now about ten known species, all of which

are deep ocean dwellers. This class is characterized by having an unhinged shell.

Class Scaphopoda

The mollusks of class Scaphopoda have a tubular shell open at both ends. There are over 400 of these species, and they are usually

found in mud or sand with the posterior end extending out. This class is unusual because it lacks both gills and heart. Therefore, gas

exchange occurs in the mantle and blood is circulated by contractions of the foot.

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Class Gastropoda are the largest group of mollusks, with between 40,000 and 75,000 species. These are also the only mollusks

that live on land. Gastropods, like class Schapoda, lack gills, and use the mantle as a makeshift lung for extracting oxygen and

breathing. Most species have coiled shells, and this class has evolved tentacles and eyes. This class includes marine animals like

the sea slug and terrestrial animals like the slug and snail.




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Class Bivalvia


Class Bivalvia (a.k.a. class Pelecypoda) includes all of the bivalves. These are marine animals with a hinged shell divided into two

halves. The hinge is head together by a ligament and one or two adductor muscles. Most live in sand or mud, and use their foot for

digging and anchoring to surfaces. These bivalves use the hinges to take in food, and are also able to jet some distance away by

closing the hinged shell and squirting the water taken in out of the mantle cavity. Bivalves lack a head and have a spacious mantle

cavity as well. Some examples include clams, oysters, scallops, and mussels.



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Class Cephalopoda


Class Cephalopoda includes marine animals that differ from other mollusks because they are built for speed and agility. They have

large brains enclosed in cartilaginous brain cases and sense organs, and most either have an internal shell, like a squid, or lack a

shell, like an octopus. All cephalopods are predators, and use jaws and radula to crush and rip prey. Like bivalves, they are able to

move by taking water into the mantle cavity and shooting it back out. Because of a developed brain, good eye sight, and sense

organs, cephalopods are considered to be one of the most advanced vertebrates.





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