Why torsion in snails

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Performance Performance. Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors. Analytics Analytics. The coil of the visceral sac and the shell, which primitively was dorsal or exogastric, becomes ventral or endogastric, after torsion.

The anus is displayed towards the right side of all pallial cavity so that the original symmetry of the organization disappears. Another characteristic feature involving asymmetry is the reduction or atrophy of the paird parts of the primitively left or topographically right side.

The remaining gill may bear one row of filaments monopectinate gill. The original advantage torsion gave gastropods is unclear. It is further complicated by the fact that torsion brought with it a number of problems. A particular problem gastropods had to overcome come was the location where wastes were excreted — above the head which can potentially lead to fouling of the mouth and sense organs. Nevertheless, the diversity and success of the gastropods suggests torsion is very advantageous indeed.

There are many other advantages torsion provided gastropods. For aquatic gastropods the anterior positioning may be useful for preventing sediment getting into the mantle cavity, which is more likely with a posterior positioning due to sediment being stirred up by the motion of the gastropod.

In terrestrial species, ventilation is better with anterior positioning. This is due to the back and forth motion of the shell during movement which would tend to block the mantle opening against the foot if it was in a posterior position. Another possible advantage for aquatic species is the osphradium olfactory sense organs are moved to an anterior position and are able to sample the water the gastropod is moving into to rather than from, this may help the gastropod locate food or avoid predators.

The changes occurring in torsion are to a certain extent reversible. This reversion is known as Detorsion and it is very characteristic of the whole group of the Euthyneura. As a result, the pallial complex travels back towards the posterior end along the right side, the ctendia point backwards, the auricles move behind the ventricle, and the visceral loop becomes untwisted and symmetrical.

In this way, a secondary external symmetry is re-established. A consequence of torsion is that the muscle scars where muscles attach to the shell are asymmetrical. Monoplacophorans are a class of mollusks like snails except they do not undergo torsion. The lack of torsion means the shell coils over the head in those having coiled shells. Furthermore, the muscle scars on their shells are symmetrical.

Few monoplacophorans survive today, but they were more plentiful millions of years ago. When you see a cartoon snail with its shell in a non-standard orientation, you might wonder if it is really a monoplacophoran? He has conducted deep-sea research for 20 years and published over 50 papers in the area.

He has participated in and led dozens of oceanographic expeditions taken him to the Antarctic and the most remote regions of the Pacific and Atlantic. Additionally, Craig is obsessed with the size of things. Sometimes this translated into actually scientific research. Fascinating article like all the other ones on this site , and the poetry is very clever and amusing.

The process of torsion. Image from the Skeptical Squirrel blog.