THE VARIED ROLES OF SNAILS - National Universities Commission

(iii) The hypertrophy of the right retractor muscle and
helicoids coiling which leads to the suppression
of the left retractor as well as to the right set of
pallial organs e.g. Trochacea.
(iv) The reason for the change in water currents and
the abandonment of the right set of the pallial
organs remains enigmatic.
(v) The hypertrophy of the post torsional right
excretory – genital duct causes the pronounced
asymmetry with the loss of the right set of pallial
organs e.g. Neritacea.
(vi) The asymmetry of the pallial organs is due to a
paedomorphous retention of the larval
asymmetry prior to the regulation in the
plantigrade stage. Owing to a long lasting
planktotrophic larval life, the development of the
right pallial organs as well as of the right
retractor muscle was more and more retarded.
The snail groups within the class gastropoda thus include
snails, limpets, slugs, whelks, conchs, periwinkles, sea
slugs, sea hares, sea butter flies, etc. These animal goups
are basically bilaterally symmetrical.
All the gastropods have common features i.e. the head,
tentacles, and at some stage of their development they
show torsion.
(a) Prosobranchia (Milne-Edwards, 1848) are
gastropods in which the adult shows torsion, the
visceral loop is in figure 8 the gills are anterior to
the heart. Common examples are Haliotis,
Patella, Buccinium, etc.
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