Phylum Mollusca, characteristics, phylum Mollusca examples, function, types, MCQ, Terminal question,

Phylum Mollusca 

Phylum Mollusca (L., molluscus, soft) includes soft-bodied invertebrate animals such as Octopods, snails, slugs, mussels, clams, oysters, tusk-shells, squids etc. The term Mollusca were coined by Johnston. It is a diverse and widespread group, with about 112,000 species. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Molluscs may be terrestrial or aquatic (freshwater or marine). They are found in all habitats - deserts, forests, lakes, rivers, abysses of sea, coral reefs, underground or even as parasite in the body of other animals.

They may be found clinging to the rocks, crawling, and swimming, burrowing or even digging. They have diversified feeding habit and can be herbivores, carnivores or omnivores. They vary in size from giant squids (nearly 2m long) to little snails, a millimeter long. The largest of the molluscan classes is Gastropods, which is represented by about 35,000 living and some 15,000 fossil species.

The three most universal features defining molluscs are - a mantle with a significant cavity used for 

breathing and excretion, the presence of a radula, and the structure of the nervous system. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, and a complex digestive system. Although molluscs are coelomates, the coelom tends to be small, and the main body cavity is a hemocoel through which blood circulates; their circulatory systems are mainly open. The generalized Mollusca has two paired nerve cords, or three in bivalves. Most molluscs have eyes, and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan reproductive system relies on external fertilization, but more complex variations occur. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults Molluscan shells have always been economically important, having served as money in early days. They have been used in jewellery and buttons. Octopus, Scallops, oysters and squids are important food items. The scientific study of molluscs is called malacology.

Habit and Habitat: -

✔Mollusca's family occur in almost in every condition every habitat found in earth, they are most conspicuous organisms.

✔Some of them found in the marine environment and some is in freshwater as well as terrestrial habitats.

✔Marine Mollusca occur on a large variety of substrates including rocks, coral reefs, mud flats, and sandy beaches. 

✔The adoption of different feeding habitat appears to have had a profound influence on molluscan evolution.

✔The change from grazing to other forms of food acquisition is one of the major features in the evolutionary of the group.

✔Truly herbivorous grazers are relatively rare and are limited to some polyplacophorans and a few gastropods groups.

General characters: - 

• Molluscs are essentially aquatic, mostly marine, few freshwaters and some terrestrial animals. 
• Body is soft, bilaterally symmetrical, unsegmented and without jointed appendages. 
• Body can be divided into head, mantle, visceral mass and foot. 
• Body is commonly protected by an exoskeletal calcareous shell of one or more pieces, secreted by the mantle. 
• The head carries mouth, eyes and tentacles. 
• The mantle is a thick, muscular fold of body wall extended over the viscera and thus enclosing a space, the mantle cavity. 
• Visceral mass contains the visceral organs of the body in a compact form.
• The foot is ventral in position and is usually thick and muscular being variously modified for creeping, ploughing and seizing but is absent in oysters.
• Body cavity is haemocoel. The true coelom is generally limited to the pericardial cavity and the lumen of the gonads and nephridia. 
• Digestive system is complete. Digestive glands are liver or hepatopancreas. Radula is mostly present. 
• Circulatory system is closed type. Heart has one or two auricles and one ventricle. 
• Respiration direct or by gills or lungs or both. The respiratory pigment is hemocyanin. 
• Excretion is performed by nephridia or kidneys. 
• Nervous system consists of paired cerebral, pleural, pedal and visceral ganglia joined by longitudinal and transverse connectives and nerves.
• Sexes are separating (dioecious) but some are hermaphrodite. Fertilization happens externally as well as internally. 
• Development is either direct or with metamorphosis through the trochophore stage called veliger larva. 

SOME EXAMPLES OF MOLLUSCA: - 

1. Octopus: -

 An Octopus (Commonly called devil fish) is a cephalopod Mollusca of the order Octopoda. This nocturnal marine creature inhabits many diverse regions of the ocean, including coral reefs, pelagic waters and the ocean floor. It is generally found on the Pacific and Atlantic coasts. Around 300 species are recognized, which is over one-third of the total number of known cephalopod species. Octopuses are among the most intelligent and behaviorally flexible of all invertebrates. Humans in many cultures eat Octopus. 

The body is unsegmented, symmetrical and soft bodied animals. Variations can be found in body parts based on species and/or geography. The head bears a pair of eyes. The mouth is surrounded by eight elongated equal arms webbed at the base and each arm bears suckers arranged in two rows. Octopus crawls on the ground and is able to change its color according to the background. The food is captured by the arms and is broken by radula and a pair of jaws. Octopus ejects ink from the ink-gland into the surrounding water, producing a smoky cloud. In males one of the arms, called hectocotylized arm, bears a spoon shaped organ at its end. The arm is used to caress the female and deposit spermatophores beneath its mantle. Sexes are separate. Development is direct. There is no larval stage. It feed upon crabs, bivalves and fishes etc.2. Sepia: -

 Sepia (Commonly known as cuttle fish) is an invertebrate belonging to the Phylum Mollusca. It is a marine animal that occupies shallow to mid water and is often associated with coral reefs. Body is 

unsegmented, bilaterally symmetrical and soft and is divisible into a large head, a small neck or collar and trunk. The head is well developed, bears a pair of eyes and 10 oral arms which are a modification of foot. Eight oral arms are smaller while two are longer and are called tentacles. The trunk extends into flaps called lateral fins on either side that help the animal in swimming. It is covered by a thick muscular mantle, enclosing a large mantle cavity, on the ventral side which contains viscera. The Sexes are separate, and reproduction is sexual. It is nocturnal and carnivorous.

3. Doris: -

 Doris (Commonly known as sea-lemon) is a sluggish marine animal found under stones at low tide marks. Head bears a pair of short retractile olfactory tentacles called rhinophores. Mantle is usually pigmented and contains calcareous spicules and tubercles. Mouth is present on ventral side. Foot is ventral with a broad sole for creeping. The body is bilaterally symmetrical, unsegmented, flat, short 

and oval with convex dorsal surface. The animal is bisexual; the penis and the genital aperture lie 

asymmetrically on the right side. The anus lies mid-dorsally near the posterior end and is surrounded by a circlet of feathery, retractile secondary gills, called cerata. Development includes a free-swimming

veliger larva.

4. Chiton: -

Chitons (formerly known as Amphineura) are marine molluscs which belong to class Amphineura and subclass Polyplacophora (fig. 4). Their size varies from small to large. About 940 extinct and 430 fossil species are recognized. Chiton is commonly known as sea-mice. Chitons are mainly found in shallow water, often under rocks and other shells, but several genera can be found in water as deep as 5,000 ft. Some are actively carnivorous, feeding on small crustaceans by smothering them with the girdle, e.g., Lepidochitona. They become active at twilight and move around very slowly in search of food. Most are grazers, and with a multi-toothed radula, they can feed on small algae and other tiny organisms.

Body is unsegmented, bilaterally symmetrical and Dorso-ventrally compressed. It consists of shell, foot, mantle and visceral mass. Shell is calcareous and is present on the dorsal side and is composed 

of 8 overlapping plates. Head is not distinct. Eyes and tentacles are absent. Foot is ventral, broad, 

sole-like and muscular, adapted for creeping and adhering. Mantle covers greater part of body and 

partly covers the edges of the shell plates. Mouth and anus are at opposite ends. Gills lie along whole length of mantle groove. Sexes are separate, gonad is single and median. Fertilization is external. Development is indirect through trochophore larva. 

5. Pecten: -

Pecten (Commonly known as scallop) is a free-swimming marine molluscs that lives on sea bottom of 10 fathoms deep. Shell is beautifully ribbed by radiating lines. The two shell valves are unequal, the right being larger and more convex and the animal rests on this valve. Locomotion takes place in spurts; the swimming is affected by the rapid opening and closing of shell valves. Foot is very much reduced. Two large gills are present. Pectens are bisexual. It is found mainly in U.S.A. and India.

6. Loligo: -

 Loligo is commonly called sea squid and is found all over the world in shallow to mid waters. Body is spindle or torpedo-shaped and divisible into head, foot and visceral hump. The head is well developed, bears a pair of eyes and ten oral arms which are a modification of foot. Eight oral arms are smaller while two are longer and are called tentacles. Shell is internal and cartilaginous. Two nephridia or 

kidneys are also present. Sexes are separate. An ink sac is present and serves for defence. Loligo is used as food by Chinese and Italians and also as bait for marine fishing.

7. Dentalium: -

 Dentalium (Commonly known as tusk shell) is a marine mollusc found world over in the sublittoral zone of seas. It measures 2-5 cm in length. It lies in a tubular, bilaterally symmetrical shell open at both ends. Head and foot project out from the anterior aperture of the shell. Head bears a mouth surrounded

by filiform tentacles called captacula. Eyes and osphradium are absent. Mantle is entirely within the shell. The foot is long pointed, spade-like and highly extensible adapted for digging and burrowing (Rastogi). Sexes are separate. Development is indirect and includes a trochophore larva. Shell of Dentalium is used for ornamental purposes. Its shells were once used by Red Indians of America as currency. It feeds on microorganisms like diatoms and foraminifera.

8. Unio: -

 Unio is commonly known as freshwater mussel. Freshwater mussels are found in freshwater lakes, rivers and streams, inhabiting the surface layers of the muddy beds of rivers and lakes. It crawls slowly with the help of its plough-like, wedge-shaped muscular foot that leaves a deep trail all along its journey. It usually moves to shallow places by night and retires to deeper places by day. Food consists of microscopic organisms, both animals and plants, which are fed upon by filter-feeding mechanism involving both ciliary as well as mucoid movements. Animal responds to light, touch or some other stimulus by withdrawing its foot and closing the siphons, meant for incoming and outgoing water currents. Soft-bodied animal is completely enclosed within a calcareous shell which represents its exoskeleton. Unio is dioecious, i.e. the sexes are separate, but there is no external dimorphism.

8. Aplysia: -

Aplysia is commonly known as sea-hare. It is a marine animal found in tropical waters. It is found in India, West Indies and on the Florida Coast. It crawls over the surface of rocks and boulders. The body is soft and lumpy with a thin flexible plate-like shell covered by mantle. The head has two pairs of tentacles. The posterior pair tentacles become ear-like and are called rhinophores. The foot is broad and flat and bears a pair of lateral folds parapodia that help in swimming. Mantle possesses unicellular ink glands. These secrete purple ink used for defense. It is bisexual. It has a single gonoduct. Aplysia is known to form mating chains with up to 20 animals. The eggs are yellow-green, and change after 8 to 9 days into a brown color before larvae hatch. The life span is assumed to be around a year. Cooler temperature delays spawning and has been shown to extend the lifespan. Sea hare is hermaphroditic, acting as male and female simultaneously during mating. Aplysia is herbivorous. Its diet consists primarily of red algae like Procambium pacificism, Ceramium eatonianum and Laurencia Pacifica which gives the animal its typically reddish or pinkish coloration. Thus Aplysia resembles the food it grazes on and cannot be distinguished easily from the seaweed unless the animal is moving.

9. Eolis: -

Eolis (Commonly known as sea slug) is a marine slug-like gastropod found in shallow waters crawling under surface of seaweeds. It is a small neudibranch 1 or 2 cm in length. Head bears two pair of cylindrical retractile tentacles and sessile eyes at the base of the posterior pair of tentacles. The mouth is prominent and can be most easily seen from the ventral view. Foot is muscular and ventral and serves for locomotion. Shell, mantle and true gills are absent. It is hermaphroditic and common gonad is protrandric. The animal is said to feed on hydroids which are digested and their undischarged nematocysts are collected in the cerata and discharged on irritation.

Important point: -

• Phylum mollusca (L., molluscus, soft) includes soft-bodied invertebrate animals such as Octopods, snails, slugs, mussels, clams, oysters, tusk-shells, squids etc. It is a very successful, diverse and widespread group, with about 112,000 species. 
• The largest of the molluscan classes is Gastropoda, which is represented by about 35,000 living and some 15,000 fossil species. 
• Molluscs can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. These classes are distinguished by, among other criteria, the presence and types of shells they possess. 
• Class Aplacophora includes worm-like animals with no shell and a rudimentary body structure. Members of class Monoplacophora have a single shell that encloses the body. 
• Members of class Polyplacophora are better known as "chitons;" these molluscs have a large foot on the ventral side and a shell composed of eight hard plates on the dorsal side. 
• Class Bivalvia consists of mollusks with two shells held together by a muscle; these include oysters, clams, and mussels. 
• Members of class Gastropoda have an asymmetrical body plan and usually have a shell, which can be planospiral or conispiral. Their key characteristic is the torsion around the perpendicular axis on the center of the foot that is modified for crawling. 
• Class Scaphopoda consists of mollusks with a single conical shell through which the head protrudes, and a foot modified into tentacles known as captaculae that are used to catch and manipulate prey.
• Molluscan shells have always been economically important, having served as money in early days. They have been used in jewellery and buttons. The scientific study of molluscs is called malacology. 
• Pila globosa is one of the largest freshwater molluscs that have invaded various kinds of habitats. It is commonly found in ponds, lakes, tanks, pools, marshes, paddy fields, streams and rivers of Northern India. The shell of Pila, as in other Gastropoda is univalve but coiled around a central axis in a right-handed spiral. It comprises of following three layers, periostracum, ostracum and hypostracum. 
• Body of Pila is divisible into four regions head, foot, visceral mass and mantle. Head is the anterior fleshy part of the body overhanging the foot. It bears mouth, two pairs of contractile tentacles and a pair of eyes. Foot is the locomotory organ of Pila and lies below the head. All visceral organs are contained in this lump like structure that lies above the head-foot complex. Skin of the visceral mass forms a thin and delicate covering called the mantle.
• The nervous system is well developed and comprises of two main parts – Central Nervous System and Peripheral Nervous System. Central Nervous System consists of ganglia and their commissures (connections between similar ganglia) and connectives (connections between dissimilar ganglia). The peripheral nervous system consists of nerves arising from CNS and innervates various parts of the body. 
• The sexes are separate in Pila and sexual dimorphism is distinct. The shell of the female is larger and more globular than that of the male. The copulatory organs or penis is well developed in male but poorly developed in female. 
• The male reproductive system consists of testis, vasa efferentia, vas deferens, copulatory organs and hypobranchinal gland. The female reproductive system consists of ovary, oviduct, receptaculum seminis, uterus, vagina, copulatory organ and hypobranchial gland. 
• Fertilization is internal but the development of the embryo takes place outside the body of female. Female lays 200-800 eggs at a time in moist earth in a sheltered cavity near ponds and lakes. The eggs are rounded and as big as the pea seeds. 
• During development, due to torsion, the visceral mass and the shell of embryo become spirally coiled. A young snail emerges from the fertilized egg. The young ones resemble the adult in form

Fill in the blanks: - 

1. Pila is popularly known as…….. 
2. Mode of respiration of Pila is ………….and……….. 
3. The study of molluscs is generally known as………… 
4. Pearl is secreted by………. 
5. Respiratory pigment in mollusca is……….

Answer:-  1). Apple snail 2). Aquatic and aerial 3). Malacology 4). Mantle 5). Haemocyanin

Multiple choice questions:- 

1. Excretory organ of Pila is: - 

(a) Kidney                                   (b) Keber’s organ 

(c) Renal organ                          (d) none 

2. The Pila is: - 

(a) Undifferentiated             (b) Hermaphrodite 

(c) Monoecious                   (d) Dioecious 

3. Aerial respiration in Pila occurs from:-

(a) Ctenidium                       (b) Pulmonary sac 

(c) Mantle                             (d) Branchiae 

4. Torsion is shown in:- 

(a) Pila                                   (b) Loligo 

(c) Unio                                 (d) Mytilus 

5. Identify the larva which is characteristic of Gastropoda and Scaphopoda: 

(a) Muller’s larva                 (b) Trochophore larva 

(c) Veliger larva                    (d) Bipinnaria larva 

6. The freshwater clam or mussel is: 

(a) Mytilus                            (b) Ensis 

(c) Anodonta                        (d) Pholas 

7. Ctenidium performs respiration is: 

(a) Air                                   (b) Water 

(c) Both the place                (d) Terrestrial 

8. Which one is a connecting link between Annelida and Mollusca? 

(a) Chaetoderma                (b) Nautilus 

(c) Neopilina                       (d) Lymnaea 

9. Concology is the study of: 

(a) Mollusca                       (b) Shell of mollusca 

(c) Foot of mollusca          (d) Behaviour of mollusca 

10. The larva of mollusca: 

(a) Glochidium                  (b) Veliger 

(c) Both                              (d) None 

Answer Key :- 

1.) c 2) d 3) b 4) a 5) b 6) c 7) b  8) c 9) b 10) c

Self Assessment Question: -

1. Classify Pila upto order? 

2. Describe the morphology of the shell of Pila? 

3. Discuss the nervous system of Pila? 

4. Describe the reproductive organs of Pila?

5.What are some representatives of the phylum Mollusca? 

6. In which habitats do molluscs live? 

7. What is the morphological feature of molluscs after which the phylum is named? 

8. What are examples of the ecological and economic importance of molluscs?