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A soft abdomen is also present in a hermit crab as an asymmetrical modification due to the asymmetrical nature of the snail shells they inhabit. [24] All gastropods are asymmetrical. This is easily seen in snails and sea snails, which have helical shells.
The evolution of an asymmetrical conispiral shell allowed gastropods to grow larger, but resulted in an unbalanced shell. Torsion allows repositioning of the shell, bringing the centre of gravity back to the middle of the gastropod's body, and thus helps prevent the animal or the shell from falling over. [2]
Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. [1] [2] [3] There are over 800 species of hermit crab, most of which possess an asymmetric abdomen concealed by a snug-fitting shell.
A common feature of the shells of mussels is an asymmetrical shell which has a thick, adherent periostracum. The animals attach themselves to a solid substrate using a byssus . A 2020 study of the phylogeny of Mytilidae recovered two main clades derived from an epifaunal ancestor, with subsequent lineages shifting to other lifestyles, and ...
Evolute shells have very little overlap, a large umbilicus, and many exposed whorls. Involute shells have strong overlap, a small umbilicus, and only the largest and most recent whorls are exposed. Shell structure can be broken down further by the width of the shell, with implications for hydrodynamic efficiency. Major shell forms include:
Mussel (/ ˈ m ʌ s ə l /) is the common name used for members of several families of bivalve molluscs, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.
The width (or breadth, or diameter) is the maximum measurement of the shell at right angles to the central axis. Both terms are only related to the description of the shell and not to the orientation of the shell on the living animal. The largest height of any shell is found in the marine snail species Syrinx aruanus, which can be up to 91 cm. [14]
A common feature of the shells of mussels is an asymmetrical shell that has a thick, adherent periostracum. The animals attach themselves to a solid substrate using a byssus . 2010 taxonomy