Phylum Mollusca : General Characters and Example

  

Defination

• Molluscs  are  soft body animals with unsegmented bodies bodies with calcareous shell.

• This is the second largest phylum of Animals after the arthropods.

General characters 

• They are terrestrial and also found in fresh and marine water.

• They  have an organ-system level of organisation. 

• They are bilaterally symmetrical, triploblastic and coelomate animals. 

• Body is covered by a calcareous shell. with unsegmented body.

• Body is divided into with a distinct head, muscular foot and visceral hump. 

• A soft and spongy layer of skin forms a mantle over the visceral hump. 

• The space between the hump and the mantle is called the mantle cavity in which feather like gills are present. 

• Gills are  respiratory and excretory functions. 

• The anterior head region has sensory tentacles. 

• The mouth contains a file-like rasping organ for feeding, called radula.

• Saxes are separated and development is indirect. 

• Mostly they are oviparous, they lay eggs.

Examples of Mollusca  

1. Octopus

• It is commonly called as Devil fish.

• Body of octopus is divided into head and Visceral hump.

• Octopus is Di branchiate having two gill , two auricle and two kidney

• Nervous system is well developed.

• Head contain eyes, siphon and eight long arms and two arms posses suckers for the attachment.

• One of the arms is male is modified into Hectocotylized  arm.

• Animal can crawl by its arms and swim backwards.

• Visceral mass and mantle cavity are enclosed by mantle.

• Shell and nidamental glands are absent.

2. Sepia

It is commonly called as culttle fish.
Body of sepia is divided into head collar and trunk or Visceral hump.
Sepia is Di branchiate having two gill , two auricle and two kidney with luminescent property.
Nervous system is well developed.
Head contain ten oral arms. eight  are long arms with several pedicellate suckers around the Mouth. And  two arms posses pedicellate suckers at the called Hectocotylized arms.


Head bear a pair of eye without olfactory crest.
Calcareous shell is well developed and maintain buoyancy.
Ink gland secrete ink during irritation and get protection from predator.

3. Loligo 

It is commonly called as culttle fish.
Body of sepia is divided into head collar and trunk or Visceral hump.
Head bear a pair of eye without olfactory crest.

Head contain ten oral arms.out of ten, eight oral arms has four row of pedicellate suckers.  

Remaining two arms has six pairs of pedicellate suckers, used in capturing of prey. These are Hectocotylised arms.


In male oral arms are modified into copulatory organ .
Loligo has two ctenidia , two auricle and two kidney.
Calcareous shell is well developed and maintain buoyancy.
Ventral siphon is modification of foot.
4. Pinctada 
It is commonly called as Pearl Oyster .
Shell valves are unequal. Hinge line is straight and bear ear and wing like process.
Left shell is large ,convex and permanently attached with the rock by byssal thread.


Right shell is small and thin covers the visceral organ.
A single adductor muscle is present.
Right and left mantle lobe are free.
Shell of animal secrete pearl when any insect create irritation.
5. Pila 
It is commonly called as Apple snail .
Body is covered by thick univalved shell and comprises body whorl , penultimate whorl and apex.
Surface of shell is marked by line of growth.
Shell is spirally coiled round the axis called as columella.
Soft parts are differentiated into head, muscular foot and visceral mass.

Head is located at the upper side with one pair of eyes.
Visceral mass contain internal organs and covered by mantle or pallium.
It has great medicinal and experimental value.
6. Aplysia 
It is commonly called as Sea hare.
Anterior end has head and neck.
Animal is slug like and anterior head are extended into two tentacular folds.


Head bear a pair of eye at anterior tentacular folds. Behind the eyes, rhinophores are present as tentacular folds.
Mantle cavity is opened at right side and backward pointing into ctenidium.
Anus is posterior located.
There are a pair large fleshy outgrowth called parapodia help in swimming.
Shell is internal and rudimentary.
6. Dentalium 
Animal is commonly called as Tusk shell.
Body is covered by arched shell secreted by mantle.
Body of animal has vestigial head which represents by proboscis without eyes.
Mouth is surrounded by leaf  tentacles called captacules which are sensory , prehensile and regenerative.
Ctenidia are absent.
Conical foot is emerged from shell to make burrow in sand.


Liver , muscle and kidney are seen after removing the shell.
Radula is well developed. Anus is located behind the foot.
Respiration takes place by lining of mantle because gills are absent.
7. Chiton 
Animal is commonly called as sea mouse.
Body is divided into district head large foot and mantle which form leaf like roof of body.
Head contain ventral mouth and labial palps.
Mouth and anus are located at opposite end of the body.
Eyes and tentacles are absent.
Dorsal side of mantle contains series of  eight  calcareous  overlapping plates.


Several pairs of bipectinate ctenidia are present on either side of body.
Side of mantle form mantle girdle which contain small calcified Spicules  .