Torsion is a gastropod synapomorphy which occurs in all gastropods during larval In some groups of gastropods (Opisthobranchia) there is a degree of secondary detorsion or rotation towards the original position; this may be only partial. Torsion. • Only gastropods undergo torsion. • Torsion moves the mantle cavity from the posterior to the Detorsion occurred in opisthobranchs and pulmonates . 3 Nov Torsion is a gastropod synapomorphy which occurs in all gastropods during larval development. Torsion is the rotation of the visceral mass.
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Visceral loop and intestine become straightened.
In simple terms, the primitive gastropod snail has twisted its body so that the posterior end, with the anus, reproductive openings, kidney openings and gills, comes to sit above the head facing forwards. The story of opisthobranch evolution is essentially the history of detorsion and the loss of the shell.
The Sea Slug Forum – Torsion and detorsion
From Wikipedia, the free encyclopedia. In Aplysia Opisthobranchiadetorsion takes place owing to the loss of shell. Despite its name, it covers all the mollusc families worldwide and has introductory chapters on all aspects of mollusc evolution.
Zoological Journal of the Linnean Society, London, 19 2: Torsion promotes stability in the adult by bringing the body of the snail nearer to substratum. Most good invertebrate text books have a general account of the process.
These cells on the right side converge and transform into the larval retractor muscles. Anybody can ask a question Anybody can answer The best answers are voted up and rise to the top.
Detorsion is brought about by reversal of the above phases. Qnd have realised in the last couple of weeks that I need to add some more of the primitive sea slugs cephalaspids in particular to the Sea Slug Forum so the evolution of the group can be better illustrated.
Torsion and Detorsion in Gastropoda
Asymmetry is encountered at the early stage in Veliger larva where the mesodermal bands develop asymmetrically. As the larva develops shell its visceral mass starts twisting in anticlockwise direction to rearrange the visceral organs so that they are accommodated inside the coils of the shell and openings of organs are shifted to the anterior side where the shell opening lies. Garstang advocated that torsion is an adaptive feature and useful to the larvae veliger larva for protection of soft parts against enemies but of little detorxion use to the adult.
In nervous system the two pleurovisceral connectives cross themselves into a figure of 8, one passing above the intestine and the other below it. The first phase occurs at a faster rate, while the next phase is slower. In specialised forms the visceral organs of one side, viz. Author are in agreement that torsion is the result of a larval mutation but they do not agree on the advantage of such mutation restricted only to the larva or extends to the adult.
Alimentary canal twists in the visceral mass and opens by anus on the side of the head on the anterior side. Then the beating of cilia stops and the larva falls to the sea bottom. Authorship details Rudman, W. There are many pelagic larvae of lamellidens which are not twisted but still survive in pelagic larval life. This page was last edited on 19 Marchat In evolutionary terms, the appearance of an operculum occurred shortly after that of torsion, which suggests a possible link with the role of torsion, though there is not sufficient evidence for or against this hypothesis.
After torsion the position becomes endogastric in which visceral mass points backwards and intestine lies in the whorls of the shell and anus opens on the anterior side. There are two different developmental stages which cause torsion. The visceral hump grows faster on one side than on the other, so that it is twisted into a compact spiral which is directed posteriorly to keep the balance of the animal, the shell is also coiled.
Before torsion the gastropod has an euthyneural nervous systemwhere the two visceral nerves run parallel down the body. During torsion the visceral mass remains almost unchanged anatomically.
Dear Manjit, I am afraid an account on gastropod torsion in general, although very interesting, is a bit outside the scope of the Forum. Description of Torsion 2. Answer Now and help others. Anybody can ask a question Anybody can answer The best answers are voted up and rise to the top. The distortion means the reversion to the changes that have occurred during torsion. In such situations twisting of visceral mass is not necessary.
Before torsion the anus and ctenidia are pointed backwards and auricles are situated behind the ventricle. The visceral hump together with the protecting shell becomes coiled to economies the volume. Here’s how it works: Both trochophore and veliger stages are passed inside the egg and a tiny snail hatches out of the egg. Because of differences between the left and right hand sides of the body, there are different evolutionary pressures on left and right hand side organs and as a result in some species there are considerable differences.
Torsion, or twisting, often lead to an assymetry in the mantle cavity organs, the loss of one of a set of paired gills being gastropodx of the most obvious examples. Check out your local university library and see what they have. Stasek and Purchon have also supported that torsion is advantageous during adult stage.