Snails, octopuses, squids and oysters are examples of molluscs.
Molluscs can be found in the sea, in fresh water and on land.
The word “mollusc” means “soft thing”. Molluscs have soft bodies and this feature explains the name of the phylum.
Because molluscs have a soft body, they are more fragile. It is also more difficult for them to support their body in terrestrial environments or to attach to substrates in aquatic habitats. Many species solve these problems by secreting a calcareous shell, which is an exoskeleton that supports them, protects them, and prevents dehydration.
The body of molluscs is not divided into segments. Considering just this aspect, they appear to be evolutionarily closer to nematodes than to annelids.
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The phylum Mollusca is divided into five main classes: pelecypods, or bivalves (Pelecypoda, or Bivalvia), which includes oysters, clams and mussels; gastropods (Gastropoda), which includes snails and sea slugs; cephalopods (Cephalopoda), which includes squids and octopuses; scaphopods (Scaphopoda), which includes tooth shells; Polyplacophora, which includes chitons. There are a few other classes of molluscs.
The body of gastropods is divided into three main portions: the head, the visceral mass and the foot.
Molluscs use extracellular digestion and contain a complete digestive system, with a mouth and an anus.
Some molluscs have a tongue-like structure with rough protrusions similar to small teeth. This structure is called the radula. The radula projects outside and helps to scrape food.
Aquatic molluscs oxygenate their blood through gills in direct contact with water. In terrestrial molluscs, the large network of veins under the mantle cavity absorbs air, playing the role of a primitive lung.
Molluscs, with the exception of cephalopods, have an open circulatory system, meaning that the blood circulates within blood vessels and through the heart, as well as filling open cavities. In annelids, the circulatory system is closed, meaning that blood circulates only inside blood vessels.
In cephalopods, the circulatory system is closed and this provides more speed and pressure for blood circulation. This permits the existence of species with large bodies, such as octopuses and giant squids.
Molluscs have one or two pairs of sponge-like nephridia, which are similar to kidneys.
Some molluscs, such as bivalves and gastropods, secrete an external calcareous shell. Calcium carbonate is secreted by the mantle, a fold in the epidermis, forming the shell.
In pelecypods, the shell is made of moving parts that contain the body between them. In gastropods, the shell only contains the visceral mass.
Octopus and squids do not produce an external shell (some species of squid can have an internal shell). One group of cephalopods, nautiluses, produces an external spiral shell.
Baby octopuses hatching!Video by Pang Quong, bit.ly/1NZ0BLMPosted by I fucking love science on Segunda, 28 de setembro de 2015
Arthropods contain an exoskeleton made of chitin. Echinoderms do not have an exoskeleton but do have a calcareous endoskeleton. Some chordates also have an external shell.
The feet of molluscs are involved in locomotion, support, attaching them to a surface, digging in the environment and sometimes even holding prey.
The endings of the names given to the main classes of molluscs are derived from the Greek word “podos”, which means foot. Gastropods have feet in their ventral region (as the name indicates); pelecypods have ax-shaped feet (ax in Greek is “pelekys”); in cephalopods, the feet are near the head.
Molluscs have well-developed sensory structures. It is accepted that cephalopods, such as octopuses and squids, have eyes with image formation. Snails have antennae and a pair of well-defined eyes. Bivalves do not have eyes but they do have photosensitive and tactile cells.
Cephalization is evident in molluscs, as they contain neurons concentrates in a ganglial pattern.
Molluscs play an important role in several food chains in ecosystems. Many marine molluscs are a part of a common human diet, such as octopuses and squids, which are very popular in Asia, and oysters and mussels, which are consumed all over the world. In addition to molluscs that are a part of the food industry, pearls made by oysters have a large commercial value.
Pearls are made from small foreign particles that are deposited between the shell and the mantle of an oyster. These particles trigger a defense process in the organism and are gradually covered by layers of calcium carbonate secreted by the oyster, thus producing pearls.
In the artificial production of pearls, a small fragment of shell covered with mantle pieces is inserted between the shell and the mantle of an oyster and a pearl is formed around the graft.
Examples of representative species: snails, sea slugs, octopuses, squids, mussels, oysters. Basic morphology: soft body, with or without a calcareous shell. Type of symmetry: bilateral. Germ layers and coelom: triploblastics, coelomates. Digestive system: complete. Respiratory system: branchial or, in terrestrial gastropods, lungs. Circulatory system: open or, in cephalopods, closed. Excretory system: nephridia. Nervous system: ganglial, advanced cephalization in cephalopods. Type of reproduction: sexual.
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