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8.5. Mollusks

Phylum Mollusca

The animals of the Phylum Mollusca are invertebrates, with a soft body and bilateral symmetry.
Molluscs form a fairly heterogeneous group but whose components are characterized by having:
  • A muscular foot. It is usually used as an organ of locomotion, like gastropods, but bivalves use it to dig or cephalopods to propel themselves.
  • Head, with mouth and sense organs, in which the radula stands out, a feeding organ formed by rows of curved chitin teeth that allow them to scrape food. It is something similar to a language but with a more complex function.
  • Mantle that surrounds the internal organs that sometimes secretes a calcareous shell that serves as support and protection for the mollusk.

Most molluscs have an internal or external calcareous shell that serves as an exoskeleton, protecting and supporting the animal. If they have an external shell, it may be made up of one or two valves. Other species, such as octopuses and slugs, do not have a shell.

Although most are aquatic, some, such as the snail, are terrestrial.

Mollusks have a complete digestive tube.

Mollusc respiration is cutaneous (through the skin) or through gills.

They reproduce sexually, they are oviparous. 

The main classes of molluscs are:

  • Gastropods.
  • Bivalves.
  • Cephalopods.

Los principales grupos de moluscos son los gasterópodos, los bivalvos y los cefalópodos

Class Gastropods

Gastropods are a class of mollusks that, as their name indicates, from the Greek gaster , "stomach" and podo, "foot" , have a foot on their belly on which they crawl.

Most gastropods are aquatic.

Aquatic gastropods breathe through gills, while terrestrial gastropods breathe through a kind of lung.

They have a heart and their circulatory system is open.

Although there are some carnivorous species, most gastropods are herbivores.

The main characteristics of gastropods are:

  • A head with a mouth, with two or four sensory tentacles, and a radula with which they feed.
  • The visceral mass. When they are larvae, gastropods are characterized by the phenomenon of torsion, in which the visceral mass rotates on the foot and the head, which allows them to hide their heads in the shell beforehand, this being an evolutionary advantage.
  • A muscular ventral foot, flattened and developed, which contains the digestive system and serves to move.
  • A single-piece (univalve) dorsal shell, usually spirally coiled, which may be reduced or lost in more highly evolved gastropods (such as slugs).

Examples of gastropods are snails, sea and land slugs, limpets, reeds, nudibranchs, etc.

Video: Reproduction of snails.

Video: How do snails move?.

Class Bivalves

Bivalves, as their name suggests, are molluscs whose body is protected by a shell that has two valves joined by an area called a  hinge. Their shells can be varied, from globular to elongated.

Bivalves are aquatic organisms, both freshwater and saltwater, and breathe through gills. They usually live at the bottom of the sea or attached to hard surfaces, such as rocks.

When the bivalves open their valves, they can take out an ax-shaped foot, with which they dig in the sand or wherever they live, and which allows them to move, such as clams and cockles. Others, on the other hand, such as mussels, live attached to rocks by means of fibers (viso).

They do not have a differentiated head and their sense organs are poorly developed.

They are filter organisms. They have a siphon through which the water enters with the organic matter that serves as food, and another through which the water is expelled after having taken the food.

Interactive activity: Characteristics of bivalves.

Class Cephalopods

The cephalopods are a class of mollusks. Its name comes from the Greek and means feet on the head, because it has a large head from which tentacles emerge. The tentacles come from the foot (characteristic of other molluscs) they have suckers and are placed around the mouth and a siphon that they use to expel water and propel themselves. They use the tentacles to capture their prey.

There are about 700 species of cephalopods, all of them marine.

Cephalopods may have an internal shell, an external shell, or no shell at all. Some, such as the Nautilus, have a septate univalved shell. Others, such as cuttlefish and squid, have an internal shell reduced to a sheet (feather). Others, like octopuses, do not.

Cephalopods are the most highly evolved class of molluscs. Cephalopods have a  highly developed nervous system with which they can carry out complex actions, such as stalking their prey or swimming backwards, expelling ink when threatened. They are the most intelligent invertebrates.

Their circulatory system is closed, which distinguishes them from other molluscs.

They are characterized by their well-differentiated head, in which the following stand out:

  • Highly developed eyes, similar to those of vertebrates.
  • They have radula, but the presence of jaws in the form of a parrot's beak with which they bite and tear the tissues of their prey while holding them with the suckers of their tentacles is more notable.

Video: Are octopuses smart?.

Crossword: Cephalopods.

Video: Squid Dissection.

Curiosity: The mimic octopus or imitator octopus or mimetic octopus

The mimic octopus (Thaumoctopus mimicus) is a cephalopod mollusk capable of imitating the shape and movements of at least fifteen dangerous animals for the predator that sees it as its future food. In this way it manages to deceive the predator and it will have to look for another prey.