The Mollusks are one group of animals considered the most diverse existing in the world, with at the least 50,000 species living. It includes such familiar organisms as snails, octopuses, squid, clams, scallops, oysters, and chitons. The group of mollusks also includes some lesser-known groups of animals such as the monoplacophores , a group that was thought to be extinct for millions of years until one was found in 1952 in the deep ocean off the coast of Costa Rica .

What are mollusks?

Mollusks are the  most diverse invertebrate animals that exist and that together make up one of the largest kingdoms in the animal kingdom , capable of surviving in a wide variety of environments .

  • Definition
  • Characteristics of mollusks
  • Types
  • Habitat
  • Nutrition
  • Reproduction
  • Breathing
  • Circulatory system of mollusks
  • Radula
  • Importance
  • Examples
  • Why are they called mollusks


The group of mollusks is a group of invertebrates that has fascinated man since the beginning of civilization. It is an important group within the animal kingdom in terms of the number it occupies with arthropods . Members of this group have adapted either on land or in water , actually inhabiting a large number of different environments . They are animals that feed by means of a radula , which is a group of curved chitin teeth, they move slowly and have a shell that provides protection.

It is important to know that at the beginning the mollusks were crawling and that over time they changed until they had a burrowing foot , a swimming foot and a split foot that became tentacles in some of them.

Characteristics of mollusks

The main characteristics that we can observe in mollusks are the following:

  • They have a smooth body that is not segmented.
  • Most mollusks have an inner or outer shell .
  • They have a kind of mantle which is a fold located in the wall of the body that divides the shell from the rest of the body.
  • Some have feet and some have tentacles
  • They have a radula , which is a toothed structure which they use to grate food.
  • They have two pairs of gills , except in snails.
  • Virtually all mollusks are divided into two functional regions: head-foot and the visceral bulge .
  • The head is the most visible part, it can be easily seen in snails and slugs.
  • Many of them are covered in cilia to help them move.


The types of mollusks that exist are the following:

Aquifer Subtype

  • Class Caudofoveata : they are vermiform, have a body full of scales, separate sexes, are marine and inhabit sedimentary bottoms.
  • Class Solenogastres : they are vermiform, their control is covered with cuticles and scales, they have a medium straight stomach, they are hermaphrodites, and they carry out internal fertilization.
  • Polyplacophora class : they are flattened, their dorsal region is covered by 8 plates, their mantle has scales and they can roll up. It has separate sexes, eggs and sperm fall freely into the water.

Conchifera subtype

  • Class Monoplacophora : their shell is conical, flattened and bilaterally symmetrical, they have modified gills, excretory organs, two pairs of gonads and a heart with two pairs of atria.
  • Class Bivalvia : they have a laterally compressed body, protected by two valves, their mouth is small and their sexes are generally separated, external fertilization.
  • Class Scaphopoda : its body is protected and is elongated, its head has two tentacles, it has a well-developed radula and it lacks gills.
  • Gastropoda class : they have variations in sizes and shapes, half of them are terrestrial, others marine and the rest live in fresh waters.
  • Class Cephalopoda : they are distinguished by the number of arms or tentacles that surround the mouth.


Mollusks are widely known within the animal kingdom for their adaptive modifications to different ways of life and environments . They are located in practically all regions of the earth and also have a great variety of forms . In addition to their abundance in habitats , they have left a continuous paleontological record since the Cambrian period . Most mollusks are aquatic and some are adapted to the terrestrial environment . In general , snails and slugs lead a terrestrial life and aquatic mollusks inhabit at leastgeneral at sea . Few bivalves and snails inhabit fresh or brackish water .


The type of nutrition varies between them, in terrestrial mollusks, such as snails and slugs we realize that they are herbivores . Marine mollusks have different types of nutrition. Some of them are filter feeders of plankton and some feed on algae. In the last case we also have animals that are predators and carnivores .


The reproduction of the mollusks can be of two types, some present a type of unisexual reproduction and the second group can be hermaphrodites , they even have the capacity of self – fertilization in some cases. Some mollusks have separate sexes and fertilization occurs externally , in the water. Some species in which fertilization occurs internally ; and in hermaphrodites , there is the presence of both sexes in the animal so they can carry out fertilization without the need for another individual, as snails do.


The exchange of gases is carried out through the surface of the body, and they use specialized respiratory organs, called gills, which reach a particular development in nudibranchs . Some types of land snails and freshwater snails, it is the mantle that has a function similar to a respiratory lung .

Circulatory system of mollusks

The circulatory system that mollusks have is open , except for cephalopods, that it is closed. The heart is septate and is divided into three chambers consisting of two atria and one ventricle . The heart is covered by a thin fabric that forms the pericardial cavity . The hemolymph passes from the ventricle and from there it travels to the vessels that distribute it through the tissue spaces , where it is collected by other vessels that go to the gills where the blood is oxygenated and from there it returns to the heart through the atrium .


The radula is a structure that is located at the base of the mouth and in the shell of mollusks . It is a structure that specializes in scraping the food that is ingested by mollusks and is also formed by a ribbon that has a number of longitudinal rows which contain small chitinous teeth . It is located in the anterior and ventral part of the mollusk and has a sac- shaped cavity known as the radular sac in which the radular apparatus is located .


Mollusks are of great importance to humans, and have been throughout history because they have been a source of food, jewelry, tools, and even pets. The shells of some mollusks are considered beautiful and valuable by many and for this reason they are also collected.


Some examples of mollusks are:

  • Octopus
  • Mussel
  • snail
  • Squid
  • Slug
  • Conch
  • Oyster
  • Sea hare
  • Periwinkle
  • Sea limpet.

Why are they called mollusks

They are called this way because the term mollusk means “soft body” , which refers to the shape and texture of these animals.

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