PHYLUM-MOLLUSCA
The phylum—Mollusca (Molluscs — soft bodied) Includes the
soft bodied, unsegmented, eucoelomate animals. These are
called molluscs or shelled animals. Johnston (1650) coined
the term ‘Mollusca’. Mollusca is the second largest animal
phylum and includes about 85000 species.
Note
The study of molluscs is called ‘Malacology’.
General Features
Some important general features of phylum—Mollusca are
(i) Habit and Habitat Molluscs are mostly of marine
forms (Sepia, Octopus, Chiton, etc.) Some are
freshwater (e.g., Unio and Pila).
These are generally bilaterally symmetrical and some
are asymmetrical due to torsion. Terrestrial form is
land snails. Few molluscs are parasites also, e.g.,
glochidium larva, etc.
(ii) Animals show symmor twisting during growth.
(iii) Germ Layers and Organisation They are
triploblastic and possess organ system level of
organisation.
(iv) Body Form They have unsegmented, soft body
covered by a calcareous shell, which is differentiated
into head, muscular foot and visceral hump.
Mantle is a soft, spongy thick fold of skin over the
visceral hump that secretes calcareous shell. The space
between the hump and the mantle is called the
mantle cavity in which feather-like gills are present.
(v) Skeleton Molluscs generally have
shell as
exoskeleton. In Octopus, shell is absent.
(vi) Body Cavity The coelom is greatly reduced
although they are eucoelomate.
(vii) Locomotion The locomotory organ is muscular foot.
(viii) Digestion The digestive tract is complete. The
mouth contains a rasping organ called radula with
file-like transverse row of chitinous teeth. Anus opens
into the mantle cavity.
(ix) Respiration In terrestrial forms, respiration occurs
through lungs. In aquatic forms, respiration occurs
through feather-like gills or ctenidia.
(x) Excretion A pair of metanephridia (kidneys) or
organs of Bojanus or Keber’s organs are present.
(xi) Circulation Open type of circulation is present. The
coelom is called haemocoel.
(xii) Sense Organs These have eyes, statocyst, tentacles for
equilibrium and receptors for touch, smell and taste.
(xiii) Nervous System Few pairs of ganglia like cerebral,
visceral and pedal with nerves are present.
(xiv) Reproduction Molluscs reproduce sexually. Sexes are
separate and mostly oviparous. Fertilisation is external
or internal.
Development is either direct or indirect with larval stages
like trochophore, glochidium and veliger. e.g., Chaetopleura
(chiton), Dentalium (elephant tusk shell),
Pila (apple snail), Pinctada (pearl oyster), Aplysia (sea hare),
Sepia (cuttlefish), Loligo (squid), Octopus (devil fish).
Knowledge Plus
• They have distinct head, much better sense organs and gills or
lungs for respiration as compared to annelids.
• Neopilina is a connecting link Annelida and Mollusca.
• Nautilus is commonly called as Nautilus. It IS used
extensively for ornamental purposes,