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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).
The adult form is without a shell or operculum (in shelled gastropods, the operculum is a bony or horny plate that can cover the opening of the shell when the body is withdrawn). In most species, there is a swimming veliger larva with a coiled shell, but the shell is shed at metamorphosis when the larva transforms into the adult form.
Unlike most other molluscs, there is no intermediate stage, or veliger, between the trochophore and the adult. Instead, a segmented shell gland forms on one side of the larva, and a foot forms on the opposite side. When the larva is ready to become an adult, the body elongates, and the shell gland secretes the plates of the shell.
The adult female colossal squid has been discovered in much shallower waters, which likely implies that females spawn in shallower waters, rather than their normal depth. [3] Additionally, the colossal squid has a high possible fecundity reaching over 4.2 million oocytes which is quite unique compared to other squids in such cold waters. [ 42 ]
Adults venture out into open water and can be found in depths up to 150 m. When mating, adults are found near coral reefs in depths of 1.5 to 8 metres (5 to 26 ft). The Caribbean reef squid is the only squid species commonly sighted by divers over inshore reefs in the Florida, Bahamas and Caribbean regions.
Adult T. gigas are the only giant clams unable to close their shells completely, allowing part of the brownish-yellow mantle to remain visible. [ 6 ] : 32 Tridacna gigas has four or five vertical folds in its shell, which serves as the main characteristic differentiating it from the similar T. derasa that has six or seven vertical folds. [ 9 ]
The class Gastropoda is a diverse and highly successful class of mollusks within the phylum Mollusca. It contains a vast total of named species, second only to the insects in overall number. The fossil history of this class goes back to the Late Cambrian .
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.