Suchergebnisse
Suchergebnisse:
The anatomy of the cardiovascular system in molluscs is highly variable, although with the exception of the Scaphopoda, all the molluscan classes have atria draining the venous blood to a central ventricular heart or hearts. This major pump propels the blood round a...
- P. J. S. Smith
- 1985
Circulatory systems in the Mollusca vary structurally from the open system typical of most classes to the closed system in the Cephalopoda. The gastropod circulatory system serves transport and exchange functions while additionally playing various hydraulic roles.
- G. B. Bourne, J. R. Redmond, D. D. Jorgensen
- 1990
Circulatory system. Most molluscs' circulatory systems are mainly open, except for cephalopods, whose circulatory systems are closed. Although molluscs are coelomates, their coeloms are reduced to fairly small spaces enclosing the heart and gonads.
Mollusks have a circulatory system with one or two hearts that pump blood. The heart is a muscular organ that pumps blood through the circulatory system when its muscles contract. The circulatory system may be open or closed, depending on the species.
- Overview
- Size range and diversity of structure
- Distribution and abundance
- Importance to humans
- GeneratedCaptionsTabForHeroSec
mollusk, any soft-bodied invertebrate of the phylum Mollusca, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body. Along with the insects and vertebrates, it is one of the most diverse groups in the animal kingdom, with nearly 100,000 (possibly as many as 150,000) described species. Each group ...
Typical molluscan features have been substantially altered, or even lost, in many subgroups. Among the cephalopods the giant squids (Architeuthis), the largest living invertebrates, attain a body length of eight metres (more than 26 feet); with the tentacle arms extended, the total length reaches to 22 metres. Other cephalopods exceed a length of o...
The mollusks have adapted to all habitats except air. Although basically marine, bivalves and gastropods include freshwater species. Gastropods have also adapted to land, with thousands of species living a fully terrestrial existence. Found on rocky, sandy, and muddy substrata, mollusks burrow, crawl, become cemented to the surface, or are free-swimming.
Mollusks are found worldwide, but there is a preponderance of some groups in certain areas of the world. The close association of many molluscan groups with their food source—whether by direct dependence on a specific food supply (e.g., plant-eating, or herbivores) or by involvement in food chains—limits their geographic distribution; for example, bivalves of the family Teredinidae (shipworms) are associated with wood. In general, cold-water regions support fewer species.
Britannica Quiz
Animal Factoids
Mollusks are of general importance within food chains and as members of ecosystems. Certain species are of direct or indirect commercial and even medical importance to humans. Many gastropod species, for example, are necessary intermediate hosts for parasitic flatworms (class Trematoda, phylum Platyhelminthes), such as the species that cause schistosomiasis in humans. Most bivalves contribute to the organic turnover in the intertidal (littoral) zones of marine and fresh water because, as filter feeders, they filter up to 40 litres (10 gallons) of water per hour. This filtering activity, however, may also seriously interfere with the various populations of invertebrate larvae (plankton) found suspended and free-swimming in the water. One species, the zebra mussel (Dreissena polymorpha), is regarded as a particularly harmful exotic invader. Carried from Europe in ship ballast water, zebra mussels were taken to the Great Lakes in 1986. To date, they have caused millions of dollars in commercial damage by clogging the water pipes of power plants and cooling systems. They are driving many native freshwater bivalve species to extinction.
Many gastropods, bivalves, and cephalopods are a source of food for many cultures and therefore play an important role in the fishing industries of many countries. Many shell-bearing molluscan species are also used to fabricate ornaments and are harvested for the pearl and mother-of-pearl industries.
Are you a student? Get Britannica Premium for only 24.95 - a 67% discount!
Learn More
Mollusks are soft-bodied invertebrates with a calcium carbonate shell, except for some cephalopods. Learn about their diversity, distribution, importance, and natural history from Britannica.
Most mollusks have an open circulatory system with a heart that circulates the hemolymph in open spaces around the organs. The octopuses and squid are an exception to this and have a closed circulatory system with two hearts that move blood through the gills and a third, systemic heart that pumps blood through the rest of the body.
Unlike other mollusks, they have a closed circulatory system, in which the blood is entirely contained in vessels rather than in a hemocoel. The foot is lobed and subdivided into arms and tentacles. Suckers with chitinized rings are present on the arms and tentacles of octopuses and squid.
