Phylum – Mollusca :
Gr. Mollis = Soft bodied
General :
The animals of this phylum are soft bodied and unsegmented. The snails, mussels, cowries and other shells found at the banks of river, ponds, lakes etc. are all molluses. The term “mollusca” was coined by Johnztone (1605). According to Mayer (1953), it includes about 80,000 species.
Salient Features :
- Body is bilateral symmetrical, triploblastic, unsegmented & soft.
- Aquatic & some are amphibious, solitary, free living.
- Presence of reduce schizocoelom.
- Body is covered by soft mental.
- Presence of exoskeleton made upfo CaCO3, which is called as shell. The shell may be internal or absent. The shell is secreted by the nacreous layer of the mantle.
- Body normally divisible into head, visceral mass and foot.
- Presence of toothed tongue called radula.
- Respiration is by gills. The amphibious forms also bear and lung-like structure.
- Blood is blue due to presence of a pigment called haemocyanin. The blood vascular system is partly open & party closed type.
- Excretion is by metanephridia or organ of Bojanus or Keber’s organ.
- Presence of simple eyes & statocyst which may be absent.
- Unisexual or bisexual, external or internal fertilization and direct or indirect development.
Classification of Phylum Mollusca :
On the basis of general characters, it is divided into 7 classes (Hyman, 1961).
1. Class – Monoplacophora
• It is a connecting link between annelid and arthropoda.
• The shell consists of only one valve (plate).
• Example – Neoplina
2. Class Aplacophora
• Absence of shell
• Example – Neomania, chactoderma
3. Class – Poly placophora
• The shell is made up of 8 plates.
• Examples – Chiton, Lepidopeurus etc.
4. Class – Scaphopoda
• The shell is tusk – shaped and open at both the ends.
• Example – Dentalium
5. Class – Gastropoda
• Some are amphibious
• Body exhibits torsion, as a result the animals become asymmetrical.
• The foot is broad & muscular.
• The larva is called as veliger.
• Examples – Pila, Cypraea, Doris, Aplysia, Limax, Helix etc.
6. Class – Pelecvpoda or bivalvia
• The shell consists of two valves which remain hinged at one side.
• The foot is plough – shaped
• It’s larva is called as Glochidium.
• Examples – Unio, Solen, Mytilus, Teredo, Pinctada
7. Class – Cephalopoda
• The shell is external or internal or absent.
• The foot divides to form oral arms and a funnel.
• The oral arms are situated on the head around the mouth. Their number is 8 or 10 or many.
• Examples – Sepia. Loigo, Octopus, Nautilus, Tethvs, Architeuthis etc.
Economic Importance of Mollusca :
The animals of phylum mollusca are of immense importance concern to food & pearl production.
• As Food – The body of many molluscs being fleshy is used as food by human beings particularly bivalians & gastropods. –
• Pearliculture – Natural pearl is an animal product and it is obtained many species of pearl oysters such as Pinctadci roding, Pinctada vulgaris, Pinctada margaritifera, mytilus etc. In India, specifically Lingha pearl is produced. The pearl is secreted by the nacreous layer of the mantle.
Chemically, the natural pearl is composed of Argonite (88 to 90%, CaCO3), Conchiolin (3.5 to 6%, Scleroprotein, C32H98H2O11). Water (2-4%) & Residue (0.1 to 0.8%). First man made pearl was produced by a Japan! scientist named Tokichi Nishikawa.