Phylum Mollusca:
Molluscs are coelomate lophotrochozoan protostomes, which develop via spiral mosaic cleavage and make a coelom
by schizocoely. The ancestral larva stage is a trochophore, but development is different in each class. They are soft
bodied and include snails, chitons, slugs, sea butterflies, clams, mussels, oysters, squids and octopuses. They can be
microscopic or as large as a giant squid. Molluscs include herbivores, predaceous carnivores, filter feeders, detritus
feeders and parasites. According to fossil evidence, molluscs originated in the sea. Molluscs are an important source
of food and their trade is of great economic importance. Molluscs survival is threatened by ocean acidification. As
the ocean becomes more acidic, levels of biologically available calcium declines, making it more difficult for marine
organisms to form calcium skeletons.
Characteristics of Phylum Mollusca:
Dorsal body wall forms pair of folds called the mantle, which encloses the mantle cavity, is modified into gills or
lungs and secretes the shell
Ventral body wall is specialised as a muscular foot used in locomotion
Aquatic and terrestrial
Free-living or parasitic
Bilateral symmetry
Unsegmented
Triploblastic
Complex digestive system
Rasping organ – radula
Anus empties into mantle cavity
Internal and external ciliary tracts
Circular, diagonal and longitudinal muscles in the body wall
Nervous system of paired cerebral,pleural,pedal and visceral ganglia,with nerve cords and subepidermal plexus
Sensory organs of touch, smell, taste, equilibrium and vision
No asexual reproduction
Monoecious and dioecious
1 or 2 kidneys (metanephridia) opening into the pericardial cavity and emptying into the mantle cavity
Gaseous exchange by gills, lungs, mantle or body surface
Open circulatory system, blood vessels and sinuses
Form and function:
- In simplest form, the mollusc body plan has a head-foot portion and a visceral mass portion
- The head-foot is the more active area, containing the feeding, cephalic sensory and locomotory organs. It
depends primarily on muscular action for its function
- The visceral mass portion contains digestive, circulatory, respiratory and reproductive organs. It depends on
ciliary tracts for its function
- Two folds of skin, outgrowths of the dorsal body wall, form a protective mantle which encloses a space between
the mantle and body wall called the mantle cavity
- The mantle cavity houses gills (ctenidia) or a lung
- In some molluscs the mantle secretes a protective shell over the visceral mass
Head-foot:
- Most molluscs have well-developed heads which bear their mouth, sensory tentacles and photoreceptors
- Radula:
A rasping, protrusible, tongue-like organ
Ribbonlike membrane bearing a surface of tiny backward pointing teeth used for feeding
Complex muscles move the radula
Radula teeth can scrape, pierce, tear and cut
Its function is to grasp fine particles of food from hard surfaces and act as a conveyer belt for carrying
particles in a continuous stream towards the digestive tract
, - Foot:
Adapted for locomotion, attachment to substratum
Waves of muscular contractions create a creeping locomotion
Secreted mucous is used as an aid to adhesion or as a slime tract for mollusc that glide on cilia
Visceral mass
Mantle and Mantle Cavity:
- The mantle is a sheath of skin, extending from the visceral mass that hangs down on each side of the body,
protecting the soft parts and creating a space called the mantle cavity
- The outer surface of the mantle secretes the shell
- Houses respiratory organs which develop from the mantle and mantle surface itself serves in gaseous exchange
- Products from the digestive, excretory and reproductive systems are emptied into the mantle cavity
- In aquatic molluscs a continuous current of water brings in oxygen and food and flushes out wastes. The mantle
is usually equipped with sensory receptors for sampling the environmental water
- Some can withdraw their head or foot into the mantle cavity, which is surrounded by the shell, for protection
Shell:
- When present it is secreted by the mantle and is lined by it
- Typically, there are 3 layers.
The periostracum is the outer organic layer, composed of an organic substance called conchiolin. It helps to
protect underlying calcareous layers from erosion. It is secreted by a fold of the mantle edge and growth
occurs only at the margin of the shell. On the older parts of a shell, periostracum often wears away
The middle prismatic layer is composed of densely packed prisms of calcium carbonate laid down in a
protein matrix. It is secreted by the glandular margin of the mantle and increase in shell size occurs at the
shell margin as the animal grows
The inner nacreous layer of the shell lies next to the mantle and is secreted continuously by the mantle
surface so that it increases in thickness during the life of the animal. It is laid down in thin layers
- The first shell appears during the larval period and grows continuously throughout life
Internal structure and function:
- Gas exchange occurs in specialised respiratory organs such as ctenidia, gills, lungs and the body surface
- Open circulatory system with a pumping heart, blood vessels and blood sinuses
- Digestive tract is complex and highly specialised. Equipped with extensive ciliary tracts
- Nervous system consists of several pairs of ganglia with connecting nerve cords. Neurosecretory cells produce a
growth hormone and function in osmoregulation
Reproduction and Life History:
- Most molluscs are dioecious, but some are hermaphroditic
- The free-swimming trochophore larva that emerges from the egg in many molluscs is similar to that in annelids
- Direct metamorphosis of a trochophore into a small juvenile is considered ancestral for molluscs
Classes of Molluscs:
Class Caudofoveata:
Worm-like marine organisms
Burrowers
Orient themselves vertically with the mantle cavity and gills at the entrance of the burrow
Feed mainly on microorganisms and detritus
Possess an oral shield, an organ associated with food intake and a radula
One pair of gills
Dioecious
Class Solenogastres:
No radula
No gills
Hermaphroditic
