Phylum Mollusca
for video https://youtu.be/oriMJdYI2K0
It is the second largest phylum of the animal kingdom.
The term mollusca was proposed by Johnston.
They are soft bodied and also possess a shell.
The study of molluscs is called Malacology and the study of molluscans shells is called chonochology.
Important characteristic features
• They are generally aquatic forms. Most of them are marine, some are found in fresh water, and a few are terrestrial. Grey slug is a terrestrial mollusc without a shell.
• All molluscs are soft bodied and unsegmented animals.
• They possess organ system level of body organisation.
• Bilaterally symmetrical except in univalves.
• They are triploblastic animals.
• They are true coelomates but the coelom is reduced in adults. The body cavity is a haemocoel.
• Their soft body is protected by an unsegmented external shell. The shell is made up of calcium carbonate, and is of one piece (univalve) or two pieces (bivalve). In some cases the shell is internal or absent.
• Body consists of four parts: head, mantle, visceral hump, and foot.
The head region has sensory tentacles.
The visceral hump contains the important internal organs. It is covered by a soft and spongy layer of skin called the mantle or pallium. Mantle secretes shell. Mantle cavity has gills.
Foot it is soft and muscular. It is the locomotory organ.
• Digestive system: It is well developed, with salivary glands and liver.
• Circulatory system: Is of the open type. Heart is chambered and valvular.
• Excretion: It has Nephridia.
• Nervous system: Consists of paired ganglia connected by nerves. Sense organs are tentacles and eyes.
• Respiration: Gills (ctenidia) in aquatic forms. Pulmonary sacs in terrestrial form.
• Reproduction: Dioecious.
Fertilization external or internal, mostly oviparous and a few viviparous. The larval forms are Glochidium, Trocophere and Veliger.
• The common molluscans are Pila (apple snail), Achatina (land snail), Lamellidens (mussel), Pinctada (pearl oyster), Sepia (cuttle fish), Loligo (squid) etc.
Classification
Class I: Polyplacophora- shell with 8 pieces. Eg: Chilton
Class II: Monoplacophora- shell with a single piece. Eg: Neopilina
Class III: Aplacophora-shell absent. Eg: Neomenia
Class IV: Scaphopoda-shell tubulr. Eg: Dentalium
Class V: Pelecypoda- shell with 2 pieces. Eg : Mussel, Oyster, Teredo, etc.
Class VI: Gastropoda- shell with one piece and spirally coiled. Eg : Pila, Snail, Slug etc.
Class VII: Cephalopoda- shell internal or reduced. Eg: Loligo, Cuttle fish, Devil fish, etc.
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