What Is the Meaning of Mollusca?
Animals come in a variety of shapes and sizes, and are divided into different groupings known as phyla. The phylum mollusca is one of the most biologically diverse groups of organisms on the planet, composing of almost 85,000 species. Mollusca are found all over the globe, from backyards to deep oceans, and have many distinct characteristics.
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Taxons
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Taxonomic classes are groups within phyla, which have very distinct characteristics. Gastropods, which include snails and slugs, are the largest of Mollusca's taxonomic classes, compromising around 80 percent of the phylum. The other large class includes the cephalopods, which are composed of animals like octopuses, squid and cuttlefish. There are numerous other minor taxons apart from the gastropods and cephalopods, such as aplacophora, bivalvia, caudofoveata, monoplacophora, polyplacophora, and scaphopoda. There are two extinct taxons, the helcionelloida and the rostroconchia.
Structure
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Many mollusca, or mollusks, have very similar body structures. They are invertebrates, meaning that they have no back bones, and often no bones at all. Mollusc's bodies are often squishy and the animals use a variety of different techniques for motion. For example, snails and slugs use a foot at the bottom of their bodies. This muscular foot serves different functions in different mollusca taxons. In snails and slugs, the foot secretes a mucus that aids in locomotion. In bivalves, such as clams, the foot secures them into sediments such as sand. In cephalopods like octopuses, the feet, which are at the bottom of their tentacles, aid in propulsion. Mollusca range in size from only an inch or so long to almost 60 feet in the case of some colossal squid.
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Shells
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Many gastropods have shells that contain most of their physical bodies. This shell is composed of chitin and conchiolin that's been hardened with calcium carbonate. This shell provides protection from predators.
Respiration and Diet
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Most mollusca undergo respiration via a single pair of gills. These gills, such as on barnacles, are typically shaped like tiny feathers and are designed to keep out sediment and other potentially harmful materials. Mollusca eat a variety of foods, from detritus to algae to live foods and crustaceans. Most gastropods use radulae, which are muscular tongues in their mouths, to bring in small food particles, with most of their mouths being lined with chitinous teeth. Mollusca such as squid and octopuses often hunt fish, using their tentacles to grip their prey and drag them into their mouths. In the case of squid, a beak in the center of their body can crush the shells of crustaceans and tear apart fish for easier swallowing.
Reproduction
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Mollusca reproduce in two ways, either through external fertilization or internal fertilization, which is composed of sexual intercourse. For example, many snails and gastropods rely on external fertilization, as do bivalves, while animals such as squid or octopuses rely on internal fertilization, with males injecting packets of sperm into the females, which she will either use or store for a future brooding. All mollusks produce eggs, which hatch into larvae that are essentially miniature versions of the adults. Some mollusks are hermaphrodites, functioning as both sexes.
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References
- Photo Credit snail image by martini from Fotolia.com
