It occurs immature in the gastropod Nassa.
The problem of the causes of the torsion of the Gastropod body has been much discussed.
According to Spengel, the pair of ganglia near the mouth, variously called labial or cerebral, represent the cerebral pair and pleural pair of a gastropod combined, and the parietosplanchnic pair correspond to the visceral ganglia, the commissure which connects them with the cerebro-pleural representing the visceral commissure.
In a dextral Gastropod the shell is coiled in a right-handed spiral from apex to mouth, and the spiral also projects to the right of the median plane of the animal.
- Four stages in the development of a Gastropod showing the process of body torsion.
B, The same, partially everted by eversion of the sides, as in the Nemertine proboscis and Gastropod eye-tentacle = pleurecbolic.
al, alimentary canal; at d, the jaws; at a, the mouth; therefore a to d is stomodaeum, whereas in the Gastropod (F) a to d is inverted body-surface.
Walcott has described a tiny lamellibranch (Modioloides) from the inferior Cambrian, and he reports the gastropod (?) genus Chuaria from the preCambrian.
The brachiopods, generally classed collectively as Spirifer mucronatus, follow at least five distinct lines of evolution in the Middle Devonian of North America, while more than twenty divergent lines have been observed by Grabau among the species of the gastropod genus Fusus in Tertiary and recent times.
Other Lamellibranchs exhibit either a trochosphere larva which becomes a veliger differing only from the Gastropod's and Pteropod's veliger in having bilateral shell-calcifications instead of a single central one; or, like Anodonta, they may develop within the gill-plates of the mother, though without presenting such a specialized 210 1P -' 1 °* larva as the glochidium.
A, Earlier, and (B), later, Veliger c, Visceral dome with dependent of a Gastropod.
This large gastropod has only been recorded in British seas on a few occasions.
A Gray Topshell (a usually abundant small gastropod) on an Oyster was unusual for this beach.
Desmoulin's Whorl Snail is a tiny gastropod that lives in swampy grasslands.
gastropod shells are of special interest due to the often elegant internal spiral structure of their shells.
molluscocky shores around the British coast are occupied by numerous marine snails, called gastropod mollusks.
The " introvert " in these Gastropods is not the pharynx The ctenidium is monopectinate and attached to the mantle along as in the Chaetopod worms, but a prae-oral structure, its apical limit being formed by the true lips and jaws, whilst the apical limit of the Chaetopod's introvert is formed by the jaws placed at the junction of pharynx and oesophagus, so that the Chaetopod's introvert is part of the stomodaeum or fore-gut, whilst that of the Gastropod is external to the alimentary canal altogether, being in front of the mouth, not behind it, as is the Chaetopod's.
Further, the Gastropod's introvert is pleurembolic (and therefore acrecbolic), and is limited both in eversion and in introversion; it cannot be completely everted owing to the muscular bands (fig.
It is rare for the gill-plume of a Pectinibranch Gastropod to stand out freely as a plume, but occasionally this more archaic condition is exhibited as in Valvata (fig.
m, Mouth; f, foot; v, velar area, the margin v corresponding with the ciliated band which demarcates the velar area or velum of the embryo Gastropod (see fig.
As in all such introand e-versible organs, eversion of the Gastropod proboscis is effected by pressure communicated by the muscular body-wall to the liquid contents (blood) of the body-space, accompanied by the relaxation of the muscles which directly pull upon either the sides or the apex of the tubular organ.
F, Acrecbolic (= pleurembolic) introvert, formed by the snout of the proboscidiferous Gastropod.
G, The acrecbolic snout of a proboscidiferous Gastropod, arrested short of complete eversion by the fibrous band b.
Other Lamellibranchs exhibit either a trochosphere larva which becomes a veliger differing only from the Gastropod's and Pteropod's veliger in having bilateral shell-calcifications instead of a single central one; or, like Anodonta, they may develop within the gill-plates of the mother, though without presenting such a specialized 210 1P -' 1 Ã‚°* larva as the glochidium.
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