Phylum Mollusca

They are the second largest animal group in the kingdom. The term Mollusca is derived from the
Latin word, ‘Mollis” - soft body. They are thought to have evolved from annelids based on the
following evolutionary similarities: a locophore larva similar to that of the polychaete worms,
central nervous system arranged in a circum-oesophageal ring, presence of perivisceral portion of
the coelom, dorsal position of the vascular trunk.

They are one of the most diverse groups of animals on the planet, with at least 50, 000 living
species (and more likely around 200, 000) and some 35, 000 fossil species. It includes such familiar
organisms as snails, octopuses, squids, clams, scallops, oysters and chitons. The living species
have been grouped into various classes according to their symmetry and the characters of the foot,
shell, mantle, gills and nervous system.

Evolution
• Fossil evidence indicates that molluscs evolved in the sea; most have remained marine

• Some bivalves and gastropods moved to brackish and freshwater

• Only snails (gastropods) have successfully invaded the land; they are limited to moist,
sheltered habitats with calcium in the soil

• The cephalopods evolved to become relatively intelligent.

• The coelom is limited to a chamber around the heart

Economic importance of Molluscs

They are economically important as a source of protein (food), sources of pearls, and other shell
materials, some are destructive to submerged woods, some introduced forms have caused
significant ecological damages, some serve as intermediate hosts for human and other vertebrate
parasites, some can cause damage to crops and shells of some species are used as jewelry and
currency.

Classification (8 Classes)
Class 1: Caudofoveata e.g. Caudofoveatans
Class 2: Solenogastres e.g. Solenogasters
Class 3: Monoplacophora e.g. Neophilina
Class 4: Class Polyplacophora e.g. Chitons
Class 5: Class Scaphoda e.g. Elephant tusk shells
Class 6: Gastropoda e.g. Snails, Limpets, Slugs
Class 7: Bivalvia/Pelecypoda e.g. Clams, Oysters, Scallops, Mussels and Shipworms
Class 8: Cephalopoda e.g. Squids, Cuttlefishes, Octopuses, Devil fishes and Nautiluses

Ancestral Molluscs

General Characteristics
 Bilaterally symmetrical, unsegmented body with a definite head.
 Ventral body wall specialized as a muscular foot, variously modified, but essentially for
locomotion.
 Dorsal body wall forms pair of folds called mantle, it encloses the mantle cavity.
 Mantle cavity modified either as gills or lungs
 Mantle cavity secretes the calcareous shell.
 Surface epithelium is ciliated bearing mucus glands and sensory nerve endings.
 The coelom is limited mainly to the area around the heart, lumen of gonads and parts of
kidney.
 Possess rasping organ – radula, anus empties into mantle cavity.
 Open circulatory system with three chambered heart, blood vessels, sinuses, respiratory
pigments.
 Gaseous exchange through gills, lungs, mantle, body surface
 One or two kidneys (metanephridia) opens into the pericardium and ends in the mantle.
 They comprise wide array of animals including gastropods (snails and slugs),
cephalopods (squids, octopuses) and bivalves (clams, oysters)..
 In general, snails are often described as those species that possess a shell into which they
can retract partially or wholly.
 Slugs may or may not have shells and for those species that do have shells, it is much
reduced and may be internal.
 For those slug species that have external shells, the shell cannot host the body of the
animal and no obvious coiling is observed.

Unique features
The Shell
The shell is one of the most important diagnostic structures in Molluscs. It has pre-viously
been employed as a basis for classi-fication. The shell may be present outside the body as in
most Gastropods, Bivalves and Chitons. Internal shell occurs in most Cephalopods and in some
Gastropods. Transitional stage, where the shell is partly internal and partly external is observed
in Hemphillia and Naticidae. In some Cephalopods and Nudibranchia, the shell is absent in
adult stage. The external dorsal shell is protective and consists of three layers, viz; outer
pericardium, middle prismatic and inner nacreous

Types of spiral
In different molluscs considerable modifications of the spiral occur. Usually the spire is more
of less obliquely coiled round the axis. Two types of spirals are usually encountered in
molluscs. The shell may be right handed – Dextral or left handed – Sinistral.

Dextral
In most of the univalve shells, the spiral is dextral meaning thereby that the spiral is right-
handed round the axis.

Sinistral
 This type of spiral does not occur commonly. This condition is just reverse to the dextral form,
i.e., the spiral is left-handed. Occurrence of sinistral type of coiling may be regarded to be
normal in certain species and in most cases sinistral type of shell represents abnormal forms.

Operculum
This is a horny plate that covers the shell aperture and it is present in almost all marine
Prosobranchia. It is a cuticular development on the foot. The operculum exactly fits into the
mouth of the shell. In all Opisthobranchia excepting Acteon and in all Pulmonata excepting
Amphibola operculum is absent. The shape of the operculum differs in different forms. The
direction of spiral does not show uniformity in all cases. In some cases, the sinistral type of
shell in the embryonic stage is converted into dextral type in adults.

Mantle
Presence of mantle is a very distinctive feature of Mollusca. The mantle cavity usually
communicates to the exterior and encloses the respiratory organs. The mantle secretes the
shelly matter and the edge of the mantle is the active productive centre. The mantle cavity is
modified in different groups of Molluscs. In Prosbranchia, the mantle cavity is situated on the
left side of the body and its anterior portion is prolonged into a tubular siphon. In Pulmonata
the mantle fold encloses the pulmonary sac. In Bivalvia mantle cavity is equally developed on
either side of the body and the mantle forms a sort of lining to the valves of the shell. Progres-
sive fusion of the edges of the mantle is observed in different bivalves.

• They have a single nephridium (kidney).

• The circulatory and nervous system are well
developed. The nervous system has three pairs of
ganglia connected by nerves.

• Sense organ includes eyes (photoreceptors,

statocyst, tactile organs & chemoreceptors)
• The eye cup contain a lens covered with a cornea.
• The osphradium situated at the base of the
incurrent siphon is sensory in function
(chemoreceptor)

Radula
The odontophore is in the mouth of most mollusks and it supports the radula (a ribbon of teeth).
In many molluscs, it moves forward while the radula contacts the food, allowing the mollusc to
feed. Mollusca can be found in freshwater, marine and terrestrial habitats. More features of
molluscs include bilateral symmetry, soft or unsegmented bodies, respiration via ctenidium,
ganglia/nerve comprised nervous system, haemocoel body cavity, etc.

o This is a rasping, protrusible, tongue like organ. It is a ribbon-like membrane on which

rows of tiny teeth pointing backward are mounted. Complex muscles move the radula and
odontophore in and out so that the membrane rotates over the cartilage tips (250,000). The
radula rasp off fine food particles and conveys it towards the digestive tract.
o The pattern and number of teeth in a row determines the speciation.

Blood
The blood system consists of a dorsal heart with one or two auricles and a ventricle. Arterial
and venous blood system is present and communication is carried out via the haemocoel. The
blood is blue because of the presence of haemocyanin, which serves as the respiratory pigment.

Nervous System
This consist of five groups of ganglia, buccal (radula), cerebral (head organ), pedal (foot),
visceral (body mass) and parietal (mantle). These are joined by longitudinal and cross-
connectives and nerves to form a circum – oesophageal ring. Sense of touch, taste or smells
are present while eye spots or complex eyes as well as statocysts for balance and osphradium
for testing water are present is some forms.

The Excretory system

In Molluscs the excretory system comprises of the kidneys and the pericardial gland. Each
kidney is a special portion of the coelom and remains in communication with other parts of the
coelom. The glandular tissue of the kidneys is arranged in various ways in different forms.

The kidneys are usually paired, symmetrical and coiled structures. The kidneys are dorsally
placed near the pericardium. One end of the kidney opens externally into the mantle cavity
while the other end opens internally into the pericardium by the reno-pericardial aperture.

Besides kidneys, there is special glandular tissue in the pericardial or viscero-pericardial

division of the coelom. This tissue constitutes the pericardial gland.

Kidneys
The kidneys in molluscs represent the special portion of the coelom. In all the Molluscs
excepting Nautilus, the kidneys communicate with the coelom. The most primitive and typical
condition of the excretory system of Molluscs are observed in Chiton. The kidneys consist of
two symmetrical glandular structures on the dorsal part of the body and are located near the
pericardium. Each kidney communicates to the mantle cavity by one end near the anus and the
other end opens into the pericardial cavity by reno-pericardial aperture. The arrangements of
the glandular tissue in the excretory organs vary greatly in different forms of Molluscs. The
number of the kidney in different Molluscs depends upon the number of auricles and ctenidia.

Reproduction
In Molluscs, the sexes may be united or separate (gonochoristic). The Solenogastres, excepting
the Chaetoderma, are all hermaphroditic. The generative organs are usually paired. In
Chaetoderma the gonads are fused together to form an unpaired one. There is no separate
gonoduct and the sexual products are discharged into the pericardial cavity. From there these
are transported to the exterior by a pair of coelomoducts.

The sexes are separate in Polyplacophora. Gonads are unpaired, sac-like structures and are
similar in appearance. The gonoducts are paired. Sexes are separate in Scaphopoda. An elon-
gated unpaired gonad is present anteriorly which narrows to form the gonoduct. The gonoduct
opens near the anus.
Sexes
In some species, the sexes are separate (Dioecious), many are hermaphroditic (Monoecious).
There may be one, two or four gonads with ducts discharging into the mantle cavity via the
pericardium and the kidneys. Fertilization is either external or internal. Most are oviparous,
while a few are viviparous. Terrestrial forms lay eggs. Development is indirect in marine and
freshwater forms via trochophore and veliger larva stages or secondary indirect

Solenogaster Veliger Larva

Trochophore Larva aplacophoran

Torsion in Gastropoda
• Torsion is a morphogenic event which occurs in gastropod molluscs as an anticlockwise
twisting of the shell and viscera relative to the head and foot of the veliger larva at a 180°
plane.
• Garstang (1928, 1929) proposed, and it has since become widely accepted, that torsion
functions as a larval defense by allowing veligers to first pull the head into the shell, then
sealing the aperture with the foot and operculum.

• Torsion has altered the bilateral symmetry of the group therefore the visceral mass has
become asymmetrical. At the beginning of a Gastropod’s life, at the tiny larval stage (the
veliger), the parts of the body begin to rotate.

• Not of all them, just the organs for digestion, reproduction, circulation, the shell, and the
body wall that covers the former and secretes the latter, twist 180˚ counter clockwise.

• This process, called torsion, eventually places the organs previously on the right side on
the left.
 • The central nervous system becomes a pretzel. Torsion quite literally ties the snail’s
stomach in knots with the whole gut eventually twisted into a U-shape.

• This gut spin also has the unfortunate consequence of placing the anus right above the
snail’s head in proximity way to close to the gills/lungs.

• Torsion occurs in stages:

• First part takes place within minutes in the veliger larva whereby the visceral mass twist
through 900

• The second 900 rotation takes a longer period.

Before Torsion
• Mantle and anus were originally posteriorly placed,

• Embryo’s mouth is anterior,

• The left gill/lung, kidney and left auricle are on the left side,

After Torsion
• Mantle cavity and anus become anteriorly placed just above the head,

• Embryo’s mouth become

• The left gill/lung, kidney and left auricle change to the right side,
• The nerve cords twist into figure 8,

• The space in the mantle cavity now houses the head when the animal withdraws within the
shell and the foot shields the entire body from the outside.

Before torsion After torsion

Consequences of Torsion to Gastropods
† A serious sanitation problem arises from the anus opening directly on top of the head and
mouth resulting in fouling
† Wastes eventually find its way back to the gills/lungs

† The osphradia a sense organ in the mantle cavity assist the animal to sample water before
the animal ventures into it.

Coiling
 Coiling can be described as the spiral winding of the shell and visceral mass.
 It was believed to have occurred at the same time with torsion.

Fossil record showed that coiling occurred earlier than torsion, but all gastropods descended from
coiled, torted ancestors.

 Early gastropods had all the whorls lined in a single plane i.e. had a planospiral shell and
were bilaterally symmetrical.

 Today, gastropods exhibit a conispiral shape i.e. each succeeding wall is at the side of the
preceding one.

 With conispiral shape, the animal had to redistribute it weight by shifting the shell upward
and posteriorly.
The shell axis now appears oblique to the
longitudinal axis of the foot.

 Weight and main body whorl pressed on

the right side of the mantle cavity.

 Loss of the gill, auricle & right kidney

leading to bilateral asymmetry.

 Fouling eventually avoided as water enters

the left and lives by the right through the
anus and nephridiopore

Ctenidia loss in Gastropoda

• The Pulmonata show some detorsion, therefore has lost their ctenidia thereby making the
vascularized mantle wall assuming a lung function by contraction of the mantle floor.

• The anus and nephridiopore opens near the pneumostome expelling wastes via water
from the lungs.

• Archachatina has two pairs of tentacles with the posterior one bearing the eyes.

Position of the eye

 Ecology of A. marginata
 They are generally nocturnal forest dwellers but have the potential to adapt to disturbed
habitats.

 They prefer concealed habitats; however, individuals may colonize more open habitats in
the event of overcrowding.

 Calcareous eggs are normally laid in the soil, are times may be deposited under leaf litter
or rocks.
 A. maginata enjoy temperature range between 20°C and 29°C.
 They shy away from the sun to avoid burning and death.
 The dark skinned snails are a common delicacy to people in West Africa.
 They are generally nocturnal forest dwellers but have the potential to adapt to disturbed
habitats. Concealed habitats are generally preferred; however, individuals may colonize
more open habitats in the event of overcrowding.
 They often become more active during periods of high humidity (e.g., after rainfall);
however, the occurrence of large numbers of individuals especially during daylight may
indicate high population density.
 They feed on both living and dead plant material. In addition to being agricultural pests.

 They can be a threat to public health as they act as a reservoir host of the rat lung parasites
(Angiostrongylus cantonensis and A. costaricensis), which cause eosinophilic
meningoencephalitis in humans.
 They can also be an unsightly public nuisance during periods of population explosion.

 A. marginata has the ability to live up to 10 years, attaining sexual maturity at 9-10 months
under laboratory conditions.

Description
 Archachatina marginata is a member of the phylum Mollusca - one of several invertebrate
animals without a spine.

 It is otherwise referred to as the giant African land snail, this is because it is the largest of
the Archachatina sub-species.
 They are native to West Africa and have a shell of up to 12-15cm in length.
 Archachatina family are giant Snails, but generally more primitive than Achatina snails.
  A. maginata has the potential to get up to 210 mm in length and 130 mm in diameter. Adult
snails have 6-7 whorls.

 The shell has a brownish yellow background with fairly uniformly arranged bands and
zigzag lines or spots that are dark-brown or reddish brown in color.
 The columella (outer lip) and inside the aperture (mouth) are white or pale blue.
 The apex of the shell is slightly flattened, bulbous and pale or pinkish in color.
 The body colour of the animal is variable (albino or tan to ash grey).

Description of the Shell

Bivalvia
Bivalve, (class Bivalvia), any of more than 15,000 species of clams, oysters, mussels, scallops,
and other members of the phylum Mollusca characterized by a shell that is divided from front to
back into left and right valves. The valves are connected to one another at a hinge. Primitive
bivalves ingest sediment; however, in most species the respiratory gills have become modified into
organs of filtration called ctenidia. In keeping with a largely sedentary and deposit-feeding or
suspension-feeding lifestyle, bivalves have lost the head and the radular rasping organ typical of
most mollusks. The shell morphology and hinge structure are used in classification. In most
surface-burrowing species (the hypothetical ancestral habit) the shells are small, spherical or oval,
with equal left and right valves. In deeper-burrowing species the shells are laterally compressed,
permitting more rapid movement through the sediments. The shells of the most efficient burrowers,
the razor clams Ensis and Solen, are laterally compressed, smooth, and elongated. Surface-
burrowing species may have an external shell sculpture of radial ribs and concentric lines, with
projections that strengthen the shell against predators and damage.
 (a) Inner view of Unio showing scars of the various muscles holding the bivalve shell in place (b)
Cross section of the shell and mantle

Cephalopoda
The cephalopods are mollusks bearing a radula (toothed tongue) and well-developed heads; mouth
characterized by a dorsoventral pair of horny jaws known as beaks and encircled by the bases of
8-ca. 60 grasping appendages; single pair of lateral, image-forming eyes; well-developed brain and
peripheral nervous system. All cephalopods are dioecious and females are generally bigger than
males. Males have one or two modified arms known as hectocotylus which are used for mating.
Fertilization takes place in the female. Development is direct to the young ones as miniature of
adult. Many species of deep-sea cephalopods occurring at depths of about 400 - 800m undergo
vertical migrations during the day and then rise into the uppermost 200 m or so during the night.
Cephalopods are carnivores and possess a radula and powerful beaks. They eat fish, crustaceans,
shellfish etc. They are major food resources for many top predators such as dolphins, whales, seals,
birds and large fish.

 The Cephalopoda (foot on

head) are marine dwellers
 They are active predators.
 They possess modified foot
concentrated on the head
region.
 The foot in cuttle fish is
modified into a funnel for
expelling water from the mantle
cavity.
 The anterior end of the margin
is drawn out into a circle or
crown of arms of tentacles.
 Size range from 2 to 3cm, some
may reach a length of about
30cm e.g. Loligo. Architeuthis
is the largest invertebrate.