Mollusca: Introduction, General characteristics and classification of phylum with examples

Introduction, General characteristics and classification of phylum Mollusca/Molluscs with examples

Introduction on Mollusca

Mollusca is the second most largest phylum of animal kingdom having over 100,000 species. Many animals that are very well known, such as: clams, snails, slugs, and squid. Also some less known animals, like: tusk shells and chitons. They mostly available in freshwater and marine environments, and some of them (molluscs) are also found on land. Molluscs living in the marine environment are probably the best known and very easily recognizable members of this phylum. Shells of many molluscs are highly valued by collectors, for example, shells from the glory-of-the-seas cone snail (Conus gloriamaris) have gained a price value of worth as high as $10,000.

It is also one of the most diverse phylum in the animal kingdom, falls second only after the arthropods in number of species, but with a wide diversity of body plan. At present there are generally seven different living classes of molluscs exists ranging from the bivalves (oysters, mussels etc) which are generally immobile with very little central nervous system, to the biggest, very intelligent species of invertebrates and the fast moving hunting squids and octopus. 

Most probably the phylum mollusca had appeared at some time in the early to middle Cambrian period, about 540 million years ago, and since most molluscs have hard calcareous shells they are quite common and important fossils. Generally most molluscs are marine inhabitant, but two of their groups the Gastropoda and Bivalvia live in freshwater and the gastropods are important inhabitant of land (as snails and slugs).

Introduction, General characteristics and classification of phylum Mollusca/Molluscs with examples

General characteristics of Mollusca

Some characteristics of phylum Mollusca are given below:- They are mostly aquatic inhabitant found in both marine and freshwater environment.

– Very few of them (i.e., molluscs) are terrestrial and if terrestrial found in moist soil.

– Organ system level of body organization is prominent in them.

– They have a true body cavity.

– The complete body of mollusc is divided into head, visceral mass, muscular foot and mantle.

– Their head (i.e., the head of molluscs) consists of tentacles and compound eyes.

– Their body is almost covered completely by a calcareous shell.

– The muscular foot in molluscs helps them in performing locomotion.

– A well-developed digestive system is present in them (i.e., in mollusks).

– They have radula which acts as the rasping organ in them for feeding.

– Respiration in them occurs through the general body surface, gills or pulmonary sac.

– The circulatory system in molluscs is of open type.

– They perform excretion through a pair of metanephridia.

– Their nervous system comprises of a number of paired ganglia and nerves.

– The sensory organs present in them are: The tentacles, eyes, osphradium, and statocysts.

– The sexes of most of the molluscs are separated but some species of molluscs are hermaphrodites too. 

– The fertilization in molluscs may either be external or internal.

РGenerally they are oviparous organisms possessing indirect development. 

Classification of Mollusca

Following are the classification of Mollusca:

Aplacophora

  • The body structure of these organisms is cylindrical.
  • They are bilaterally symmetrical organisms.
  • These organisms are devoid of head, shell, mantel, nephridia, and foot.
  • They are devoid of digestive ceca.
  • Almost the whole body of these organisms is covered with spicule-bearing cuticle.
  • They have a dorsal longitudinal keel or crest.
  • E.g. Neomenia, Chaetoderma

Monoplacophora

  • Their body is bilaterally symmetrical.
  • Their head does not bear eyes and tentacles.
  • Respiration in them occurs through their gills which are externally located.
  • They excrete out the nitrogenous waste from their body through nephridia.
  • E.g. Neopilina

Polyplacophora

  • The body of these organisms is dorsoventrally flattened like that of a leaf.
  • They are bilaterally symmetrical organism.
  • The shell of these organisms comprises of 8 longitudinal plates.
  • A well-developed radula is present in them.
  • The ventral foot present in them is flat.
  • E.g. Chiton, Cryptochiton.

Gastropoda

  • They are aquatic and terrestrial organism as they are found either on land or in fresh and marine water.
  • Their head comprises of tentacles, eyes, and a mouth.
  • The shell of these organisms is spiral in shape.
  • They have flat and large foot.
  • E.g. Haliotis, Pila

Scaphopoda

  • They are exclusively marine inhabitant.
  • They do not bear eyes and tentacles.
  • They have reduced foot.
  • They have bilaterally symmetrical body.
  • E.g. Dentalium, Dentaliida

Pelecypoda

  • They are aquatic inhabitants.
  • Their body is bilaterally symmetrical and also laterally compressed.
  • Pelecypoda usually lack a distinct head.
  • They usually burrow inside mud and sand.
  • E.g. Mussels, Unio

Cephalopoda

  • These organisms are mostly abundant in the marine environment.
  • The shell of these organisms is either external, internal, or not present at all.
  • These organisms have separate sexes.
  • The development of these organisms is direct.
  • E.g. Octopus, Spirula

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