The word Mollusca is derived from the Latin word molluscus meaning soft. Mollusca are the animals whose body is soft, asymmetrical or bilaterally symmetrical, segmentless, ciliated, fleshy legs and exoskeleton made of calcium carbonate. Mollusca is the second largest phylum of fauna. But it is the largest marine episode. 23% of the ocean is molluscs. Precious pearls, Tyrian purple dye, sea silk, lime etc. are obtained from animals of this phase. The giant squid of this episode is 12 meters long and weighs 270 kg. Land snails (Achatina fulica) and snails cause extensive damage to crops and are known as crop pests. The number of species in this category is about 1,00,000. Johnston (1650) named the order Mollusca.
Characteristics of the Mollusca
1. Their bodies are soft, fleshy and unsegmented.
2. They are cylomates, asymmetrical or bilaterally symmetrical and have a conspicuous head.
3. At the base of the body are muscular fleshy legs.
4. The body is covered by a thin covering called mantle. Calcareous material secreted from the mantle forms the shell.
5. Respiration occurs by gills and mantle membrane. Pulmonary sacs are developed in terrestrials.
6. The digestive tract consists of sand-tongues or radulas made of chitin.
7. The body cavity is very short and turned into a hemocele. Circulatory system is semi-free type.
8. The alimentary canal is straight, curved or U-shaped.
9. Blood contains hemocyanin (meat and copper) and amoebocytes.
10. Siloam is limited only to the sides of the heart, the genital cavity and the kidneys.
11. A unisexual or bisexual animal. They have sex.
12. The life cycle consists of trochophore, velizar or gochidium larvae.
13. They live in sea salt water, fresh water, ditches or pits.
Classification of Mollusca
The order Mollusca is divided into 5 classes.
1. Caudofovita: They are 2–140 mm long. Body covered with chitinous cuticle. They are marine.
2. Polyplacophora: They are 8 mm to 33 cm long. Body covered by chitin.
3. Gastropods: Their bodies are soft, fleshy and unsegmented. Body covered by mantle.
4. Bivalvia: They have hemolymph fluid. There are two shells made of calcium carbonate.
5. Cephalopoda: These are marine animals with eight arms. There is no hard shell on the body. Has eight legs (cephalopods). They are nocturnal.
Some animals of Mollusca
Snail – Pila globosa
Octopus – Octopus macropus
Mussel – Lamellidens marginalis
Dentalium – Dentalium kernedei
Loligo – Loligo edulis
Cattle fish – Sepia aculeata
Neopilina – Neopilina galatheae
Teredo – Teredo navalis
Mussel – Unio marginalis
Pearl oyster – Pinctada vulgaris
Chiton – Chiton tuberculatus
Chiton – Ischnochiton evanida
Chaetoderma – Chaetoderma nitidulum
Solen – Solen ensis
Garden Snail – Helix pomatia
King Scallop– Pecten maximus
Devil fish
Eight armed marine animals belonging to the Mollusca family are called devil fish. It is called Octopus. They grab prey by their arms and kill them with poisonous saliva secreted from their mouths. For example – Octopus macropus.
Cuttle fish
Sepia and Loligo belong to Mollusca group called cuttlefish. Cuttlefish are edible animals. Their body is shield-shaped. Shells exist on the body. For example – Sepia aculeata.
Radula
The radula is a sand tongue. The radula or sand tongue is a toothed tongue-like organ in the digestive system of molluscs. It is made of chitin. It helps in taking and chewing food. For example – Snail (Pila globosa).
Ctenidia
Pteridia are respiratory organs. The gill-like breathing organs in the molluscs are called taenia. It resides in the mantle or mantle cavity or on the body surface of animals.