THE VARIED ROLES OF SNAILS - National Universities Commission

ight larval retractor i.e. establishment of the first
phase of torsion (90%).
(d) The predominant development of the
pretorsional right (or the retarded development of
the pretorsional left), pallial organs due to
respiratory currents.
(e) Regulation of the divergent axial and balanced
conditions between the visceral hump and the
head foot respective of plantigrade movement
which is by differential growth processes in
metamorphosing animals. That is the second
phase of torsion of approximately another 90 o .
(f) This regulation includes and is combined with
the development of the second (post torsional
right) set of pallial organs including the retractor
muscle, the mantle or shell sinus or slit enabling
the new paired inhalant respiratory current to be
directed symmetrically from the latero-frontal
areas towards the anterior-medio-dorsal area.
In the larval stages of gastropods the internal organs
undergo twisting as the animal develops. For example, the
digestive tract is simply U-shaped but during torsion it
completes a 150 o twist that brings the anus up over the
mouth. This twist causes other organs on one side of the
body to be compressed and fail to develop while the
nervous system contorts to a figure eight.
Among other groups after the torsion there is a counter
clockwise detorsion, moving the pallial cavity to a position
on the right side of the body, behind the heart. This
situation can be found among the opisthobranchia (hind gill
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