1. PHYLUM MOLLUSCA BY MRS OZEMOKA H.J
PHYLUM MOLLUSCA by MRS OZEMOKA H.J. is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

2. PRE- TEST What are Annelids?
State the differences between the three classes of Annelids.
State the economic importance of Annelids.
What is mollusk?
State the characteristics of molluscs.
State the economic importance of mollusk.
Name the classes of mollusk you know.

3. INTENDED LEARNING OUT - COME
At the end of the class, the students are expected to know the following:
What molluscs are
The characteristics of Molluscs
The various classes of Molluscs
The distinguishing characteristics between the various classes
The economic importance of Molluscs
The difference between molluscs and other phyla.

4. INTRODUCTION
The word Mollusk, refers to soft body. They are thus soft bodied animals. It is the second largest group of living organisms which are represented by a number of species which are more than 100,000 living species. It is a very diverse group which includes slow moving snails and slugs, relatively sedentary bivalves such as clams, and highly active cephalopods such as squids,Octopus etc. It ranges from almost microscopic organisms to giant squid. The Scientific study of mollusc is called MALACOLOGY.

5. CHARACTERISTICS

7. CHARACTERISTICS OF MOLLUSCS
They are soft bodied, triploblastic, unsegmented coelomates which are bilaterally symmetrical.
They possess a mantle which secretes the shell.
They possess a radula which is toothed belt/ ribbon for feeding
They have open circulatory system (secondarily closed in Cephalopods) which consist of a three chambered heart which expand into an extensive haemocoel.
The respiratory pigment is haemocyanin.

8. CHARACTERISTICS CONTINUES
The coelom is reduced and limited to areas around the heart, lumen of gonads, part of kidney and occasionally part of intestine.
They are cosmopolitan(found in the marine,fresh water,and terrestrial habitat).
They are free living and occasionally parasitic.
They are oviparous.The egg hatches into a free swimming trochophore larva and later metamorphose into a veliger larva or miniature adult.

9. CHARACTERISTICS CONTINUES

10. CLASSIFICATION OF MOLLUSC
This classification is based on structure of shell and radula. There are seven classes namely:
Aplacophora (Shell-less): Solenogastres and Caudofoveata
Monoplacophora (One plate bearer)
Polyplacophora (many plates bearer)
Bivalvia (Pelycopoda)
Cephalopoda (Head foot)
Gastropoda (Belly foot)
Scaphopoda (Tusk shell or tooth shell)

11. EXAMPLES OF MOLLUSC CLASSES

12. APLACOPHORA Small worm like. Shell is absent.
Foot if present folds in the pedial groove
Respiration is by gills or secondary gills.
Nephridia is absent.
Nervous system is ladder like.
Development could be direct or indirect
Examples are Neomenia, Chaetoderma etc.

14. MONOPLACOPHORA
They have an undivided arched shell. The shell is limpet –like.
Have broad flat foot.
Serially repeated pairs of gills(3- 6 pairs) and retractor muscles.
They are thought to have gone into extinction. They are living fossils. A stricking feature is the serial repetition of internal organs in several systems eg Neopilina.
In 1952, they were discovered or collected from the deep sea of the Pacific Coast of Costa Rica

15. POLYPLACOPHORA
The shell is divided into eight transverse shell valves or shell plates.
Foot is broad and flat.
Gills are 6-8 pairs.
Nervous system is ladder- like but lacks ganglia
Development is indirect and the trochophore larva metamorphoses directly into adult without the veliger larva.
They flattened dorso- ventrally and have a convex dorsal surface which bears the eight articulating plates.
Examples include: Chiton, Lepidopleurus etc.

17. BIVALVIA They are filter feeders and as such lack radula.
They are called bivalves because they possess a two part shell that is hinged and can be closed by strong muscles.
Excretion is carried out by siphon.

18. CEPALOPODA
They are referred to as head – foot. They include octopus, squid, nautiluses, devil fish, cuttle fish etc.
They are all marine and active predators(efficient predators).
Largestest and most complex mollusk.
Shell is often reduced, internal or wholly absent.
Foot is modified to form part of head and tentacles.
Highly developed head with tentacles and suckers as well as well developed eyes
The anus is posteriorly located.
Possess a radula and horny beak.

21. CHARACTERISTICS OF GASTROPODS
The largest and most diversed molluscan group.
Includes snails, limpets and slugs.
The members are cosmopolitan.
They are characterized by TORSION and COILED SHELL.
They may be monoecious or diecious.
They produce an external spirally coiled shell except for slugs.

22. TORSION
It is the 180 degrees anticlockwise (counter- clock wise) twisting of the visceral mass, mantle and mantle cavity. It positions the gills, anus, the reproductive and excretory openings just over the head . It twist the digestive tract into a “U” shape. Torsion occurs during veliger stage.

24. ADVANTAGES OF TORSION
It ensures that the head is withdrawn into the shell first before the foot. Once the foot is drawn in, an operculum closes the opening of the shell.
Anterior opening of the mantle cavity ensures that only clean water enters the mantle cavity rather than silt contaminated water which is stirred up by the snail’s crawling activity.
The twist in the sensory organ makes it more sensitive to stimuli coming from the direction in which it moves.

25. PROBLEM OF TORSION It causes fouling problem of the head.
TACKLING THE PROBLEM
Some gastropods undergo detorsion in which the larva or embryo undergoes a full 180 degrees torsion and then untwist approximately 90 degrees. The mantle cavity now opens on the right side of the body behind the head.
Various modifications allows the water and waste to exit the mantle cavity through openings in the mantle and shell posterior to the head.

26. DETORSION

28. CHARACTERISTICS OF THE VARIOUS SUB- CLASSES OF GASTROPODS
PROSOBRANCHIA
OPISTHOBRANCHIA
PULMONATA
About 20,000 species
Fewer than 2,000 species
About 17,000 species
Mostly marine with few fresh water or terrestrial
Mostly marine
Fresh water or terrestrial species
Most members are herbivores or deposit feeders while some are carnivores.
They are predators. They prey on cnidarians.
They are herbivore with a long radula for scraping plant materials.
Foot is modified into an undulating fin which propels the animal through water.
Foot is modified into thin lobes for swimming
Foot for crawling on surfaces
They use nematocysts which they acquire from their prey to ward off predators.
Highly vascularized mantle cavity which serves as lungs.
Example: Heteropod(Carinaria)
Sea slugs, Sea hares etc.
Snails and slugs.

29. SCAPHOPODA(BOAT FOOT)
They are referred to as tooth shells or tusk shells.
There are over 300 species.
Exclusively marine and are all burrowing organisms which inhabit moderate depths.
They possess a conical shell which opens at both ends.
The head and foot projects from the wider side of the shell.
They exhibit tentacles for feeding and defense.
They lack functional gills and as such gaseous exchange occurs across the mantle cavity.

30. ILLUSTRATION OF SCAPHOPODA

32. ECONOMIC IMPORTANCE OF MOLLUSC
As source of food.
Source of income.
Intermediate host for some tremadoes such as Schistosomes.