Phylum Mollusca Characteristics

The largest marine phylum, having around 93,000 species, molluscs, have a great variety in terms of anatomy, habitat, and behavior. Most molluscs are categorized under the class 'gastropoda'. The major gastropods are snails and slugs. The word mollusc is derived from the French word 'mollusque'.

This French word too was derived from the Latin word 'molluscus' which means 'soft'. The habitats of these creatures include the oceans and freshwater, and they form an important part of the marine biome. Some of these creatures, however, are also terrestrial. Let us have a look at the various characteristics of this phylum.

Snails, clams and squids are some of the popular examples of this phylum. The facts related to the anatomy, physiology etc., of these creatures are enlisted in the story.

The anatomy/structures of molluscs are complex in nature. One of the special features of their body structure is the presence of a coelom.

The coelom is a cavity present in body of molluscs, which contains internal organs suspended from mesenteries. Let us find out more information about the anatomical characteristics of molluscs.

• The upper surface of molluscs is covered by a shell formed by the secretions of mantle. A muscular foot is present on underside of the body.
• The mantle cavity is a body part of molluscs formed by the fold produced in the mantle. The exact location/position of the mantle varies from one species to the other.

• The different organs present in the mantle cavity include the reproductive organs (gonads), gills, kidneys (nephridia) and the anus. The anus is formed of chemical sensors called osphradia.

• The shells of molluscs are formed of calcium carbonate (95%) and organic components (1%). Biomineralization is a special feature of mollusc shells which give them the necessary strength.

• The feet of the molluscs carry out different functions in different classes. A balance organ called statocyst is incorporated in the foot structure of molluscs.

• In limpets, the foot is modified into a sucker which helps in anchoring the animal to hard surfaces.
• The foot in the gastropods secrete mucus to facilitate movement.

The facts presented below reveal interesting information about the characteristics of phylum mollusca that are physiological in nature.

Locomotion
There is great diversity among molluscs in terms of locomotory organs. Most of the molluscs move from one place to other by means of muscular contraction. The herbivorous species glide over waves of water. The undulating lateral fins of the cuttlefish enables it to move by performing maneuvers. A jet propulsion technique is used by cephalopods for swimming rapidly in water.

Digestive and Excretory Functions
The stomach of molluscs have a complicated structure. The mouth and the anus form the parts of the digestive and excretory system; these parts are connected to each other by the complex stomach. The entire digestive tract is lined by cilia.

Absorption of food from the digestive tract is done by cells aligned with digestive glands in the stomach; nutrients absorbed from this food, then enters the blood. Waste or undigested materials are excreted through the anus.

The waste materials before excretion are compressed and packaged in a solid form. Before reaching the anus these undigested materials are stored in the mantle cavity.

Phylum Mollusca Reproduction
Reproduction of molluscs takes place by means of external fertilization, through which eggs are produced. There is a lot of variation in the types of larvae that emerge from eggs. Larvae could be the trochopore, veliger or in some cases even miniature adults are produced directly.

Nervous and Sensory System of Molluscs
Molluscs have a highly developed nervous system which varies from species to species. The octopus has such a complex and intelligent brain that its mental abilities match that of a domestic cat.

Thus, it is deemed as the most intelligent invertebrate. Squids are capable of locating transparent creatures by means of polarized vision. The giant squid is known to have biggest eyes in the entire animal kingdom.

There is a lot to say about molluscs. Variety in their forms, body features, etc., has been studied to reveal interesting information about them.