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Molluscs have developed such a varied range of body structures, finding synapomorphies (defining characteristics) to apply to all modern groups is difficult. [15] The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical. [16] The following are present in all modern molluscs: [17] [19]
Members of the molluscs, annelids, platyhelminths and nemerteans have all been shown to exhibit spiral cleavage in its classical form. Other spiralian phyla (rotifers, brachiopods, phoronids, gastrotrichs, and bryozoans) are also said to display a derived form of spiral cleavage in at least a portion of their constituent species, although evidence for this is sparse.
The mollusc (or mollusk [spelling 1]) shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes. Not all shelled molluscs live in the sea; many live on the land and in freshwater.
The sculpture of the valves is one of the taxonomic characteristics, along with the granulation or spinulation of the girdle. [ 12 ] After a chiton dies, the individual valves which make up the eight-part shell come apart because the girdle is no longer holding them together, and then the plates sometimes wash up in beach drift.
In many species of molluscs the epidermis of the mantle secretes calcium carbonate and conchiolin, and creates a shell. In sea slugs there is a progressive loss of the shell and the mantle becomes the dorsal surface of the animal. The words mantle and pallium both originally meant ‘cloak’ or ‘cape’; see mantle (vesture).
The taxonomic term Bivalvia was first used by Linnaeus in the 10th edition of his Systema Naturae in 1758 to refer to animals having shells composed of two valves. [3] More recently, the class was known as Pelecypoda, meaning "axe-foot" (based on the shape of the foot of the animal when extended).
Malacology [a] is the branch of invertebrate zoology that deals with the study of the Mollusca (molluscs or mollusks), the second-largest phylum of animals in terms of described species [1] after the arthropods. Mollusks include snails and slugs, clams, and cephalopods, along with numerous other kinds, many of which have shells.
The mouth is located within the animal's undeveloped head in front of its single large foot and contains a radula, a defining characteristic of the mollusca. Tentacles are situated behind the mouth. They also have a cone-shaped stomach with a single crystalline style though no gastric shield. The intestines are long and make between four and ...