The size of molluscs varies greatly. The largest body form is the giant squid 18 m in length. The smallest garden slug is less than 1 cm long. They have following common characteristics:
- Body has two parts: head-foot and visceral mass.
- Mantle secretes a calcareous shell. It covers the visceral mass.
- Mantle cavity functions in excretion, gas exchange, elimination of digestive wastes. and release of reproductive products
- They have bilateral symmetry.
- The develop protosotme characteristics like trochuphore larvae, spiral cleavage, and schizocoelous coelom formation.
- Coelom reduced to a cavity. This cavity surrounds the heart, nephridia, and gonads.
- They have open circulatory system except in one class (Cephalopoda).
- Radula is present. It is used in scraping of food.
Regions of the body
The body of molluscs has three main regions: the head- foot, the visceral mass, and the mantle.
(a) Head- foot region: It is elongated with an anterior head. Head contains mouth and certain nervous and sensory structures. It has elongated foot. It is used for attachment and locomotion.
(b) Visceral mass: Visceral mass contains the organs of digestion, circulation, reproduction, and excretion. It is present dorsal to the head-foot.
(c) Mantle: Mantle of molluscs is attached to the visceral mass. It enfolds most of the body. It secretes a shell that overlies the mantle.
Th shell of the molluscs is secreted in three layers:
1. Periostracum: The outer layer of the shell is called the l’eriostracum. It is protein in nature. Mantle cells at the mantle’s outer margin secrete this layer.
2. Prismatic layer:
The middle layer of the shell is called the prismatic layer. It is the thickest of the three layers. It consists of calcium carbonate mixed with organic materials. Cells at the mantle’s outer margin also secrete this layer.
3. Nacreous layer: The inner- layer of the shell is called nacreous layer. It forms thin sheets of calcium carbonate alternating with organic matter. The epithelial border cells of the mantle secrete the nacreous layer. Nacre secretion thickens the shell.
Th space between the mantle and the foot is a called the mantle cavity. The mantle enmity opens to the outside. It functions in gas exchange. excretion. elimination of digestive wastes and release of reproductive products.
The mouth of most molluscs possess a rasping organ called radula. Radula consists of a chitinous belt and rows of posteriorly curved teeth. The radula lies over a fleshy tongue like structure. It is supported by a cartilaginous odontophore. Muscles and odontophore help the radula to protrudeout from the mouth.
Muscles move the radula back and forth over the odontophore. Food is scraped from a substrate and passed posteriorly to the digestive tract.