THE VARIED ROLES OF SNAILS - National Universities Commission

(b) Opisthobranchia are gastropods in which the
adults show detorsion by a process of untwisting
e.g. Aplysia, Doris, etc.
(c) Pulmonata are gastropods in which the adult
nervous system becomes symmetrically
developed following torsion by a process of
shortening of the abdominal commissures e.g.
Achatina, Lymnaea Bulinus,
Biomphalaria, etc. (see table 1)
Table 1 : Comparative Study of the Morphology of
the 3 Sub-Classes
S/n Characters Prosobrachia Opisthobranchia Pulm
1. Torsion The visceral mass
exhibits torsion to
a maximum
degree
2. Ctenidia Primitive forms
have two gills.
Advanced forms
have one i.e.
monopectinate
3. Mantle Cavity The mantle cavity
is anterior and
hence the name
Prosobranch. The
opening is wide.
Undergoes
detorsion with
Gut straight
One or more or
none secondary anal
gills may be present
When present
cavity opens on the
right side by a wide
aperture
30
Com
Gut
Ctn
The
ante
is n
pneu
roof
and
bein
lung
4. Auricles/Kidne Two auricles and One auricle and one One