PHYLUM MOLLUSCA

The animals belonging to the phylum Mollusca have soft-bodies, triploblastic and bilaterally symmetrical and coelomate. The study of Mollusca is called Malacology. They are sluggish invertebrates, with a thin fleshy envelope or mantle covering the visceral organs.

 

The term Mollusca was derived from the term given by Aristotle to cuttlefish. Mollusc means soft. These organisms are found in the terrestrial as well as in deep seas. Their size ranges from microscopic organisms to organisms 20 metres long. They play a very important role in the lives of humans. They are a source of jewellery as well as food. Natural pearls are formed within these molluscs. The bivalve molluscs are used as bioindicators in the marine and freshwater environments. But few of them such as snails and slugs are pests.

Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not just in size and anatomical structure, but also in behaviour and habitat. The phylum is typically divided into 7 or 8 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish, and octopuses, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs and account for 80% of the total classified species.

 

The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula (except for bivalves), and the structure of the nervous system. Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc" (see image below). This has a single, "limpet-like" shell on top, which is made of proteins and chitin reinforced with calcium carbonate, and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are coelomates, the coelom tends to be small.

 

 The main body cavity is a hemocoel through which blood circulates; as such, their circulatory systems are mainly open. The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, and a complex digestive system in which exuded mucus and microscopic, muscle-powered "hairs" called cilia play various important roles. The generalized mollusc has two paired nerve cords, or three in bivalves. The brain, in species that have one, encircles the esophagus. 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. Nearly all produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. The coelomic cavity is reduced. They have an open circulatory system and kidney-like organs for excretion.

The most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system. Many have a calcareous shell.Molluscs have developed such a varied range of body structures, finding synapomorphies (defining characteristics) to apply to all modern groups is difficult. The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical.

 

Examples of Phylum Mollusca

Following are a few examples of the animals belonging to phylum Mollusca:

1.      Pila (Apple snail)

2.      Limax (Slug)

3.      Unio (Freshwater mussel)

4.      Turbinella (Shankha)

5.      Helix (Garden snail)

6.      Octopus (Devilfish)

7.      Loligo (Squid)

 

Mollusca Characteristics

The organisms belonging to phylum Mollusca exhibit the following characteristics:

1. They are mostly found in marine and freshwater. Very few are terrestrial and found in moist soil.
2. They exhibit organ system level of organization.
3. Their body has a cavity.
4. The body is divided into head, visceral mass, muscular foot and mantle.
5. The head comprises of tentacles and compound eyes.
6. The body is covered by a calcareous shell.
7. The muscular foot helps in locomotion.
8. They have a well-developed digestive system, the radula is the rasping organ for feeding.
9. They respire through the general body surface, gills or pulmonary sac.
10. The blood circulates through the open circulatory system.

 

 

Classification of Mollusca

Following are the classification of Mollusca:

a.      Aplacophora or Solenogasters

                           i.      The body is cylindrical or bilaterally symmetrical.

                         ii.      These are devoid of the head, shell, mantel, nephridia, and foot.

                       iii.      The digestive ceca is absent.

b.      Monoplacophora

                        i.         The body is bilaterally symmetrical.

                      ii.         The head is devoid of eyes and tentacles.

                    iii.         Respiration occurs through gills which are externally located.

c.       Polyplacophora

                     i.            Their body is dorsoventrally flattened like a leaf, and are bilaterally symmetrical.

                   ii.            The shell is composed of 8 longitudinal plates.

                 iii.            They have a well-developed radula.

                 iv.            The ventral foot is flat.

d.      Gastropoda

                           i.      They are found either on land or in fresh and marine water.

                         ii.      The head bears tentacles, eyes, and a mouth.

                       iii.      The shell is spiral in shape.

e.       Scaphopoda

                     i.            Found in the marine environment.

                   ii.            The eyes and tentacles are absent.

                 iii.            The foot is reduced.

f.        Pelecypoda

                     i.            They reside in aquatic habitats.

                   ii.            The body is bilaterally symmetrical and compressed laterally.

                 iii.            The body has no distinct head.

g.      Cephalopoda

                     i.            They are mostly found in the marine environment.

                   ii.            The shell is either external, internal, or not present at all.

                 iii.            They have separate sexes.

                 iv.            The development is direct.