Phylum: Mollusca, its classification and characteristics
- multicellular, triploblastic, system grade of body organization.
- mostly bilaterally symmetrical except gastropods.
- mostly marine, some are found in fresh water whereas some are terrestrial.
- body is soft and unsegmented.
- body is divided into head, foot and visceral mass.
- body enclosed in shell formed of calcium carbonate.
- shell may be external, internal, reduced or absent.
- locomotion by muscular foot which is modified for creeping, swimming and burrowing.
- complete alimentary canal and grasping organ radula is present in buccal cavity except pelecypoda.
- haemocoelomate, coelom is reduced to pericardial cavity and cavities of gonads and kidneys.
- respiration by ctenidia (gills), pulmonary sac in terrestrial forms.
- excretion by metanephridia.
- tentacles, eyes, statocyst (balancing organ), osphradium (water testing organ), etc are present as well developed sense organs.
- mostly unisexual, few are bisexual.
- development is direct or indirect.
Image source: onlinescience
Classification
Molluscs are divided into six classes on the basis of nature of foot and shell. They are:
Class 1: Monoplacophora
- mostly marine.
- body is segmented enclosed in sub circular shell with spirally coiled protoconch.
- head without eyes and tentacles.
- locomotion by flat ventral muscular foot.
examples: Neopilina (connecting link between annelida and mollusca)
Image source: pininterest
Class 2: Polyplacophora (Amphineura)
- exclusively marine.
- body is enclosed by shell consisting of eight transverse rows of oval or longitudinal plates (valves).
- head reduced without eyes and tentacles.
- locomotion by thick, flat, sole like ventral foot.
examples: Chiton, Chitoderma
Class 3: Gastropoda
- freshwater or marine or terrestrial, also found in moist soil.
- body is covered by spirally coiled shell.
- distinct head with eyes and tentacles.
- adult gastropods are asymmetrical due to torsion.
Examples: Pila globosa (apple snail), Limax (grey slug), Helix (garden snail)
Class 4: Scaphopoda
- mostly marine.
- body is enclosed in tubular shell open at both ends.
- head and eyes are absent.
- conical foot used for burrowing.
examples: Dentalium (tusk shell), Cadutus
Class 5: Pelecypoda (bivalvia)
- mostly marine, few are found in fresh water.
- body is enclosed in bivalve shell which are joined mid dorsally.
- head, eyes, radula and tentacles are absent.
- hatchet shaped foot (claw like foot).
examples: Unio (fresh water mussel), Mytilus, Pinctada (pearl oyster)
Class 6: Cephalopoda
- mostly marine.
- distinct head with large eyes and tentacles.
- foot is modified into arms and tentacles with sucker.
- shell is external, internal, reduced or absent.
examples; Octopus (devil fish), Loligo (squid), Sepia (cuttle fish)
Image source: quizlet
Reference:
i) https://manoa.hawaii.edu/exploringourfluidearth/biological/invertebrates/phylum-mollusca
ii) https://www.toppr.com/guides/biology/animal-kingdom/phylum-mollusca/