Slide 1 : Dr Subroto Biswas Mollusca
Slide 2 : Johnston (1650) coined the word "mollusca",
Second biggest phylum
Includes 1,12000 species.
Unique…. : Unique…. Shell
Enclosed in mantle or pallium
Open circulatory system
Molluscs include : Molluscs include the chitons,
tooth shells,
snails, slug,
nudibranchs,
sea butterflies,
clam mussels,
oysters,
squids,
octopuses and
nautiluses.
MOLLUSCA : MOLLUSCA True coelomate schizocoelous animals.
Protostomes have spiral cleavage
Determinate mosaic dvelopment.
Organ systems present
Simple organisms to most complex invertebrates
Slide 6 : Microscopic to the giant squid, Architeuthis.
This huge molluscs may grow to 18 m long, including their tentacles.
They may weigh 450 kg (1000 pounds).
Slide 7 : Many molluscs have a trochophore larva similar to marine annelids.
Thus indicates that molluscs and annelids share a common ancestor.
As molluscs are not metameric, they must have diverged from their common ancestor with the annelids before the advent of metamerism.
Slide 8 :
General Characteristics : General Characteristics Triploblastic,
Schizocoelomate
Bilaterally symmetrical animals.
Gastropods are asymmetrical due to torsion.
Live in fresh water, marine water and some live in terrestrial environment.
Pila globosa is an amphibious gastropod.
Body is unsegmented
But monoplacophoran molluscs show internal segmentation.
Slide 10 : The body divisible into… head, foot and visceral mass.
Body is covered by mantle or pallium(thin fold).
Below it the mantle cavity secretes the shell.
Shell protects the soft-body parts.
The shell is made by calcium carbonate and conchiolin.
Slide 11 : These animals have no appendages.
They have smooth muscles (form of bundles).
Coelom is reduced.
Confined to kidney, pericardial space and gonads.
Digestive system is well developed,
A rasping organ in the buccal cavity …radula is present
Bivalves have no radula.
Hepatopancreas a big gland is associated with alimentary canal.
Slide 12 : Respiration by ctenidia or pulmonary sac or by both.
Blood vascular system is open type.
Blood flows in sinuses.
In cephalopoda, members have closed blood vascular system.
Blood contains a respiratory pigment called haemocyanin.
Slide 13 : Excretion by metanephridium.
Nervous system is well developed
Shows paired ganglia and connectives.
Sense organs include, eyes, statocyst and osphradium (osphra means smell)
Osphradium is leaf like structure. It tests the chemicals of water.
Slide 14 : Most of them are unisexual animal. Some are bisexual.
Fertilization is external or internal.
Development is direct or indirect.
In their life history, they may show trochophore larva or veliger larva or glochidium larva.
Glochidium larva leads ectoparasitic life in the beginning and then becomes independent.
It is seen in the life history of Unio.
Diversity of Mollusca(seven classes) : Diversity of Mollusca(seven classes)
Class : Aplacophora : Class : Aplacophora Primitive and worm-like animals
Shell, foot and gills are absent.
Head is distinct
No eyes and tentacles.
Mantle is present.
Heart has one auricle and one ventricle.
Radula is present in the buccal cavity.
Statocyst is absent.
They have one pair of gonads.
Examples : Chaetoderma and Neomenia.
Chaetoderma is a worm like shell-less marine mollusc : Chaetoderma is a worm like shell-less marine mollusc
Class : Polyplacophora : Class : Polyplacophora Dorsoventrally flattened
Shell with 8 dorsal calcareous plates.
Head indistinct
Eyes and tentacles are absent.
Radula is present in the buccal cavity.
Mantle is present
Many pairs of ctenidia present in the mantle cavity.
Heart shows two auricles and one ventricle.
One pair of metanephridia performs excretion .
Sexes are separate.
Veliger larvae is seen in the life cycle.
E.g., Chiton, Lepidofileurus and Chaetopleura.
Chiton is popularly called 'coat of mail shell’ and has 8 dorsal plates : Chiton is popularly called 'coat of mail shell’ and has 8 dorsal plates
Class : Monoplacophora : Class : Monoplacophora Body is dome shaped
Indistinct head .
Shell is broad (single piece).
Eyes and tentacles are absent.
Foot is flat and broad ( ventral side)
Body is covered by mantle.
5 to 6 pairs of gills
6 pairs of nephridia are present.
They show internal segmentation.
Connecting link between annelida and mollusca
Statocyst is absent.
They have only one pair of gonads.
Examples : Neopilina.
Neopilina, the living fossil is the only known surviving members of the class monoplacophora of molluscs.It lived during the Cambrian period.It has repeated nephridial units (six pair) suggesting its primitive and evolutionary relationship between molluscs and annelids. : Neopilina, the living fossil is the only known surviving members of the class monoplacophora of molluscs.It lived during the Cambrian period.It has repeated nephridial units (six pair) suggesting its primitive and evolutionary relationship between molluscs and annelids.
Class : Scaphopoda : Class : Scaphopoda ‘Elephant tusk shells’ are included in this class.
Body is elongated.
Covered by a single piece of shell which opens at both the ends
Head is absent.
Mouth bears tentacles.
Near the mouth long thin lobed structure are present. They are called captacula. They are useful in feeding and respiration.
Radula is present
Foot is triangular and useful for boring.
Mantle is present.
Gills are absent.
Paired nephridia are present.
Statocyst is absent.
Sexes are separate.
Trochophore larva is seen in the life cycle.
Examples : Dentalium and Pulsellum.
Dentalium is known as 'elephant's tooth' : Dentalium is known as 'elephant's tooth'
Class : Gastropoda : Class : Gastropoda Snails and slugs are included in this class.
Shell is made of single piece (spirally coiled ).
Asymmetrical because of torsion at the time of development.
Head is distinct with eyes and tentacles.
Radula is present.
Foot is broad and flat.
Respiration is by single ctenidium or pulmonary sac or both.
Excretion is by single kidney. It is called organ of Bojanus.
Statocyst is present.
They are unisexual or bisexual animals.
They show single gonad.
Development includes veliger larva.
Examples :
Pila globosa (apple snail), Doris (sea lemon), Aplysia (sea hare), and Cypraea (Cowry)
Members of gastropoda with coiled shells : Members of gastropoda with coiled shells
Slide 26 :
Members of gastropoda : Members of gastropoda
Garden snail : Garden snail
Aplysia a sea hare swims with parapodia and secretes a liquid from its purple gland to protect itself : Aplysia a sea hare swims with parapodia and secretes a liquid from its purple gland to protect itself
Limnaea (pond snail) : Limnaea (pond snail)
Class : Pelecypoda (bivalvia or lamellibranchiata) : Class : Pelecypoda (bivalvia or lamellibranchiata) Body is laterally compressed.
Shell shows two valves, hence the name bivalvia
Head, eyes, radula and tentacles are absent.
Foot is ‘wedge shaped’ hence the name pelecypoda.
Respiration is by one pair of ctenidia.
Excretion is by one pair of kidneys. Keber's organ is also an excretory organ.
They have statocyst.
They show one pair gonads.
Their life cycle includes veliger or glochidium larva.
Examples include Unio (fresh water mussel), Mytilus (sea mussel), Pecten (scallop), Ostrea (edible oyster), Pinctada vulgaris (pearl oyster)