General Characters of Phylum Mollusca
The phylum Mollusca is composed of variety of organism. They includes slow moving snails and slugs bivalved (shell with two parts) clams and highly active cephalopods. This phylum has 50,000 living species. It is the second largest phylum of invertebrates. Giant squid is the largest invertebrate animal. Molluscs show a great variety of form (structure). But all Molluscs have same basic structure. They show following characters:
- Body Characters: They are triploblastic, coelomate and show bilateral symmetry.
- Mantle and shell: The body is covered by a glandular (having glands) epithelial envelope called mantle. It secretes a calcareous shell. The shell is a protective structure. However, it slows down the locomotion. So some of the active Molluscs have reduced or lost the shell.
- Habitat: They are widely distributed. Some groups are exclusively aquatic, freshwater or marine (e.g. Cephalopoda). The other are Molluscs are terrestrial animals (e.g. Land snail). They live in moist places.
- Body structure: They have unsegmented and soft body. The body can be divided into
- A ventral muscular foot
- Dorsal visceral mass, containing most of the internal organs.
Mantle is present over the visceral mass. Mantle secretes outer shell. The space between shell and the mantle cavity contains gills in some Molluscs. A rasping tongue like structure radula is present in the mouth cavity of many Molluscs. Radula has many horny teeth. The body is highly organized. It has complex digestive, respiratory, circulatory, excretory, nervous and reproductive systems.
- Digestive system: The digestive system consists of gut. It has two opening, mouth and anus.
- Excretory system: The excretory organs are paired nephridia.
- Circulatory system: They have open circulatory system, except Cephalopoda. The coelom is divided into sinuses or blood spaces.
Heart pumps the blood into these sinuses. They have blue respiratory pigment called haemocyanin.
- Respiratory system: The exchange of gases takes place by gills. In some cases like snail, the mantle cavity is converted into lungs.
- Nervous system: The nervous system consists of three pairs of interconnected ganglia. These ganglia are present in head, foot and body region.
- LocomotIon: The organ of locomotion is a muscular foot. Movement is slow in many species. The other Molluscs are sessile i.e. unable to move.
- Reproduction: The sexes are separate. They develop trochophore larva during their development.
Classification of Phylum Mollusca
The Molluscs are divided into six classes. There following major classes:
- Their body is asymmetrical (without any symmetry). Their body is covered with coiled shell. The animal can withdraw itself into shell.
- They are both aquatic and terrestrial.
- The aquatic species have lungs. While in the terrestrial animals the mantle cavity is converted into lungs.
- They have bilateral symmetry with dorso-ventrally body.
- All species are aquatic.
- The shell is much reduced and internal. In most cases, it is absent.
- These animals are highly developed and active.
- Loligo: Squid
- Sepia: Cuttle Fish
Economic Importance of Mollusca
Molluscs are indirectly harmful to man. The harmful Molluscs are slugs and shipworms.
- Slug: They are harmful for gardens and other cultivations. They eat leaves and destroy plants by cutting up their roots and stems.
- Teredo (Shipworm): It damages wooden parts of ships.
Most of the Molluscs are beneficial.
- Many types of Molluscs are used as foods by man. A large quantity of clams, oyster (cf) and mussels are eaten in Far East (Japan, China), Europe and America. Oysters are regard as delicious food.
- Shells of freshwater mussels are used in button industry.
- Shells of oysters are mixed with tar for making roads in America.
- Their shells are used for making ornaments in certain countries.
- Some oysters make valuable pearls. For Example: Pearl Oyster.