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Diversity[ edit ] Diversity and variability of shells of molluscs on display. They include snailsslugs and other gastropods ; clams and other bivalves ; squids and other cephalopods ; and other lesser-known but similarly distinctive subgroups.
The majority of species still live in the oceans, from the seashores to the abyssal zonebut some form a significant part of the freshwater fauna and the terrestrial ecosystems. Molluscs are extremely diverse in tropical and temperate regions, but can be found at all latitudes.
Estimates of the numbers of nonmarine molluscs vary widely, partly because many regions have not been thoroughly surveyed. There is also a shortage of specialists who can identify all the animals in any one area to species.
For comparison, the great majority of mollusc species are marine, but only 41 of these appeared on the Red List. Mollusc shell Anatomical diagram of a hypothetical ancestral mollusc Because of the great range of anatomical diversity among molluscs, many textbooks start the subject of molluscan anatomy by describing what is called an archi-mollusc, hypothetical generalized mollusc, or hypothetical ancestral mollusc HAM to illustrate the most common features found within the phylum.
The depiction is visually rather similar to modern monoplacophorans.
|Zobodat - Suchergebnisse||Introduction The Mollusks are a large and diverse group of soft-bodied unsegmented animals.|
|Phylum Mollusca | An Overview of Biodiversity||From the dark abyss to shallow tide pools, research has recently revealed some of the mysterious behaviors of two famed cephalopods, the Giant Squid and the deadly Blue-ringed octopus click image below for an enlargement.|
The shell is secreted by a mantle covering the upper surface. The underside consists of a single muscular "foot". It contains the body organs. It is lined with epidermis, and is exposed, according to habitatto sea, fresh water or air. The cavity was at the rear in the earliest molluscs, but its position now varies from group to group.
The anusa pair of osphradia chemical sensors in the incoming "lane", the hindmost pair of gills and the exit openings of the nephridia "kidneys" and gonads reproductive organs are in the mantle cavity.
Mollusc shell The mantle edge secretes a shell secondarily absent in a number of taxonomic groups, such as the nudibranchs  that consists of mainly chitin and conchiolin a protein hardened with calcium carbonate  except the outermost layer, which in almost all cases is all conchiolin see periostracum.
In gastropods, it secretes mucus as a lubricant to aid movement. In forms having only a top shell, such as limpetsthe foot acts as a sucker attaching the animal to a hard surface, and the vertical muscles clamp the shell down over it; in other molluscs, the vertical muscles pull the foot and other exposed soft parts into the shell.
Although molluscs are coelomatestheir coeloms are reduced to fairly small spaces enclosing the heart and gonads.
The main body cavity is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficient hydrostatic skeleton. The heart consists of one or more pairs of atria auricleswhich receive oxygenated blood from the gills and pump it to the ventriclewhich pumps it into the aorta main arterywhich is fairly short and opens into the hemocoel.
A pair of nephridia "little kidneys" to the rear of and connected to the coelom extracts any re-usable materials from the urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity.
Respiration[ edit ] Most molluscs have only one pair of gills, or even only a singular gill. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side.Cephalopod: Cephalopod, any member of the class Cephalopoda of the phylum Mollusca, a small group of highly advanced and organized, exclusively marine animals.
The octopus, squid, cuttlefish, and chambered nautilus are familiar representatives. Learn more about cephalopods in this article. Three main clades are typically identified within the Cephalopoda (cladogram B, left): the Ammonoidea (an extinct and shelled clade), Nautiloidea (with only one living shelled genus, Nautilus) and Coleoidea (squids, cuttlefish, octopuses, the Ram's Horn Squid, the "paper nautilus," and an extinct clade, the belemnites).
The ammonoid lineage survived for million years in the oceans of the Paleozoic and .
Enrico Schwabe, Jorge Holtheuer, Dirk Schories (): First record of a mesoparasite (Crustacea, Copepoda) infesting a polyplacophoran (Mollusca, Polyplacophora) in Chilean waters, with an overview of the family Chitonophilidae – Spixiana, Zeitschrift für Zoologie – - Mollusca Overview.
Bilateral symmetry, protostomy Trochophore larvae, COELOM REDUCED, circulatory system, mantle, radula What kind of reproduction does Cephalopoda have?
(Squid_ Chitons (look like trilobite) How many dorsal valves does the trilobite polyplacophora have? 8 dorsal valves. What kind of head does a polyplacophora have.
- Very strong muscular foot in the Polyplacophora (Chitons) allows them to attach to rocks Mantle - In all mollusc lineages, a mantle covers the internal organs of the visceral mass.
Volborthella, some fossils of which predate, was long thought to be a cephalopod, but discoveries of more detailed fossils showed its shell was not secreted, but built from grains of the mineral silicon dioxide (silica), and it was not divided into a series of compartments by septa as those of fossil shelled cephalopods and the living Nautilus are.