Phylum Mollusca

Activity 7

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Phylum Mollusca• Mollusk means soft-bodied.• A mollusk has no internal skeleton but many have shells that

act as an external skeleton, providing protection. • Most mollusks usually have some version of a head, soft body,

and foot. • One important part of a mollusk's body is the mantle, which is

the fleshy tissue that lines the inside of the shell. – This part is responsible for shell growth and color, and it assists in

other functions such as respiration. – Growth of the shell occurs by the mantle absorbing calcium

carbonate from the sea water. – The color of the shell is due to pigment cells in the mantle.

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• (Latin.mollis = soft)

• over 80 000 species. • soft, unsegmented body, consisting of an anterior head, a dorsal

visceral mass and a ventral foot.• The body is more or less surrounded by a fleshy mantle (an

outgrowth of the body wall) and nearly all species in the group secrete a lime shell that covers and protects the body.

• All, except the class Bivalvia, have a ribbon-like rasping tongue (radula - unique to this phylum) with small chitinous teeth that processes the food.

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• Most mollusks are free living, but slow moving creatures, showing a close association with the substrate.

• Some attach to rocks or shells, others burrow, others float, octopuses and squids swim freely.

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Classes of Mollusca

• Bivalvia • Gastropoda • Cephalopoda• Aplacophora• Monoplacophora• Polyplacophora• Scaphophoda

(http://coa.acnatsci.org/conchnet/c-101a2.html)

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Characteristics of Mollusc

• Body usually short and partially or wholy enclosed by a fleshy outgrowth of the body wall called the mantle, which may be variously modified. Between the mantle and the visceral mass is a mantle cavity containing components of several systems (secondarily lost in a few groups).

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• A shell (if present) is secreted by the mantle and consists of one, two or eight parts.

• The head and the ventral muscular foot are closely allied (the foot being variously modified for burrowing, crawling, swimming, or food capture).

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• The digestive canals are complete and intricate with ciliary canals for the sorting of particles.

• The mouth with a radula bearing traverse rows of minute chitinous teeth to rasp food , except in Bivalvia.

• The anus opening in the mantle cavity. A large digestive gland and often salivary glands are present.

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• The circulatory system is open, except in Cephalopoda and usually includes a dorsal heart with one or two atrias and one ventricle. This is situated in a pericardial cavity. An anterior aorta and other vessels and many blood spaces (hemocoels) exist in the tissues.

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• Respiration occurs via one to many uniquely structured ctenidia (gills) in the mantle cavity (secondarily lost in some), by the mantle cavity, or by the mantle.

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• Excretion by kidneys (nephridia), one or two or six pairs, or only a single one. They usually connect to the pericardial cavity and they exit in the mantle cavity.

• The coelom is reduced to the cavities of the nephridia, gonads and pericardium.

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• The nervous system is typically a circumesophageal nerve ring with: – multiple pairs of ganglia and – two pairs of nerve cords (one pair innervating the

foot and another the visceral mass). • Many poses organs for smell, or touch, or taste. • Eyespots or complex eyes present. • A statocyst for equilibration present.

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• The sexes are usually separate(some are monoecious, a few are protandric).

• Gonads add up to four, two or one, all with ducts. • Fertilization occurs externally or internally. • Most species are oviparous. • Egg cleavage determinate, spiral, unequal and total

(meroblastic in Cephalopoda). • Trochophores and veliger larvae form, or a parasitic stage

occurs(Unionidae), or the development is direct (Plumonata, Cephalopoda).

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• Unsegmented (except Monoplasophora).

• Symmetry bilateral or asymmetrical.

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Class POLYPLACOPHORA

• Polyplacophora ("many plate bearers")

• contains the Chitons – easily recognizable because of

their shells that are split into eight dorsal plates that cover the centre of their bodies.

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Class GASTROPODA

• Gastropods, which include univalves, are mollusks usually covered by a single shell,

• Some gastropods, such as slugs, have no shell at all. Many of the subclass Opisthobranchia (sea slugs, sea hares, nudibranches etc.) have forsaken their gills and shells.

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Class GASTROPODA• The class Gastropoda is the largest in the phylum and

includes the snails, whelks, winkles and sea slugs.

Subclass PROSOBRANCHIA = Snails, limpets

Subclass OPISTHOBRANCHIA = Sea slugs

Gastropod anatomy

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Class GASTROPODA

• Most of these species belong to the subclass, Prosobranchia, and have a spiral shaped shell, a well developed head that poses tentacles and a radula, and a large flat foot for motion.

• The primitive members are herbivores that rasp seaweeds and micro algae.

• Around 40 000 known species.

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Class GASTROPODA

• The shelled gastropod body emerges from an opening, called an aperture, to eat and move. The outer edge of the aperture is called a lip.

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Class GASTROPODA

• Each coil of a gastropods shell is called a whorl, with the last and usually the largest whorl containing the body, thus called the body whorl. All whorls above the body whorl make up the spire.

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The calcareous layers of the shell are covered by a thin, glossy, proteinaceous periostracum.

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Gastropod means stomach-foot because the foot is large and

is the most prominent feature in most cases. The rest of the

body basically contains the stomach, which rests on the

foot.

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• As with bivalves, most gastropods breathe by taking in oxygen from the water through siphons.

Univalves, unlike bivalves, have a head with tentacles, which have sense organs that can detect shadows and movement.

Anterior pair of tentaclesshorter

Posterior pair tentacleslongereyes at distal tip

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• Many gastropods have a trap door, or operculum, attached to their foot, which is actually a thin piece of shell.– This operculum seals the

aperture closed when the animal retreats into its shell, providing protection from predators and from drying out.

operculum is a thin teardrop-shaped disk of flexible protein

(conchiolin)

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Class GASTROPODA

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Class BIVALVIA (Pelecypoda)• Includes:

– Mussels– clams – oysters

• Has a shell that consists of two parts, but unlike the Brachiopoda (Lamp shells) the two parts are hinged together dorsally and then extends downwards, laterally on either side of the body, thereby encasing it. Interior view of right valve showing

the muscle scars.

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Class BIVALVIA

• Bivalve means two shells or valves. – These two shells are attached at a hinge where one

or more strong muscles inside the shell keep it tightly closed, and a rubbery ligament (hinge ligament) near the hinge holds the halves together and open.

– Scars can sometimes be seen on the inside of some shells where the muscles were once attached.

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Concentric linesRecede from an elevated point near the hinged margin

Umbo

(UMBO)

Approximating points of umbos

of opposite valves

Beak

Ridges/folds or spines

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Interior view of right valve showing the muscle scars.

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Interior view of right valve showing the muscle scars.

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Pallial sinus

Pallial line

Point of attachment of pallial retractor muscle to the shell.

A P

V

Serrations Insertion area of mantle

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Cardinal teeth

Lateral teeth

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Class BIVALVIA

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Class BIVALVIA

• Most of the bivalves poses large gills for the purposes of respiration and filtering out of small food particles.

Bivalve with left valve and mantle removed.(Heavy arrows - path of water current, Dashed arrow - path of filtered particles).

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Class BIVALVIA

• Most are sedentary suspension feeders– Depend on ciliary currents produced by the gills to

bring food materials.

• Some are deposit feeders

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Class BIVALVIA

• A bivalve breathes by circulating water within its shell, which brings in oxygen. As the water leaves the shell, it carries with it carbon dioxide and other wastes.

• Some bivalves have siphons, where water enters and leaves through.

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Class BIVALVIA

• Bivalves lack a radula.

• one or two pairs of gills(ctenidia) or branchia for respiration.

• Many species burrow in the sand or mud and poses large, wedge-shaped feet for this.

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Class BIVALVIA

• Mussels attach to rocks by means of a beard-like byssus, while oysters and their allies cement one of their valves to the rock face.

• Around 20 000 known species.

Section through the visceral mass showing the internal organs.

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Class BIVALVIA

Clam anatomy

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Inside of the right shell showing scars where muscles are attached

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Class BIVALVIA

A

P

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mantle

Gills (Ctenidia)

Byssal threads

Foot

Anterior adductor muscle

Posterior adductor muscle

Anterior retractor muscle

Posterior retractor muscle

Visceral mass

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Class CEPHALOPODA

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Class CEPHALOPODA

• Octopus, cuttlefish and squid

Dorsal view of a squid (loligo) in swimming position.The tentacles and arms are held together and functions as a rudder.

Cuttlefish (sepia) seizing a shrimp with the use of its tentacles.

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Class CEPHALOPODA

• Instead of a foot they poses eight or ten long tentacles armed with suckers.

Anatomy of a squid - loligo (ventral view with the mantle cut open).

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• Cephalopods are divided even further into the eight-armed octopods (octopuses) and the 10-armed decapods (cuttlefish and squid).

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Class CEPHALOPODA• Most of the cephalopods are active and predatory swimmers

posesing jaws and radula.

• This class includes the deep-sea giant squids (20m long) which were the inspiration for many a mythical story about sea monsters.

• The Cephalopods poses eyes as complex as those of humans, and a greater capacity for learning than any other invertabrates.

• Around 650 known species.

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Sagittal section of nautilus.

Nautilus : the only shelled cephalopod.

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Reference:

• http://library.thinkquest.org/26153/marine/mollusca.htm

• http://oceanica.cofc.edu/shellguide/molluskframeset.htm

• http://www.applesnail.net/content/anatomy/respiration.php

• http://www.applesnail.net/content/anatomy/shell.php

• Hickman Jr. C.P. and et al., 2007. Animal Diversity 4th edition. Boston: McGrawHill