What animals are mollusks?

molluscs (edge Mollusca) are one of the largest groups of animals on the entire planet, the second among animals invertebrates and the first among Marine animalsalthough they have also colonized habitats of sweet water and even habitats terrestrial.

It is estimated that they appeared about 500 million years ago, time that they have invested in developing all their great biodiversity. They know each other around 75-85 thousand extant species at present, although they could exceed 100 thousand.

Although they are characterized by shell and a muscle foot, not all have a shell and the foot may be highly modified. Includes animals like snails, octopus or clams.

General characteristics

The body of molluscs has two-sided symmetryis not segmented and is softcharacteristic that gives them their name: Mollusca means "soft" in Latin.

Apart from that soft body, molluscs have a great morphological diversityalthough the general description of molluscs is usually done based on a hypothetical mollusk model putative ancestor of modern molluscs.

Anatomy model of a mollusk
Anatomy model of a mollusk

The three anatomical features that define the current molluscs are:

  • Presence of mantle or palliuman anatomical structure that they use to breathe and excrete. The mantle covers part of the body and forms the mantle cavity or pallial cavity which houses the gills.
  • The anus and gonads open into the mantle cavity..
  • General structure of the nervous system with a periesophageal ring of ganglia and two pairs of nerve cords.

The position of the mantle varies greatly from one mollusk to another. In some, such as the squid, the mantle extends as layers beyond the body to increase the size of the pallial cavity; the fins and the outer body wall of the squid forms part of the mantle.

Other molluscs, such as bivalveshave practically their entire body wrapped in a enlarged mantle.

In the mollusks with shellthe mantle is also the organ responsible for secrete proteins and calcium carbonate that form the shell.

In other groups of molluscs, instead of a shell, epidermal spicules or platessometimes only present in the embryonic stage and not in adults.

The epidermis of molluscs usually has hair cells and produces large amounts of bugger.

Another very characteristic organ of molluscs is the muscle footalthough it has evolved very differently in the different groups of molluscs.

In modern molluscs we can find the creeping foot of snails, the digging foot of bivalves, the swimming foot of many gastropods or the foot divided into tentacles of cephalopods such as the octopus, the squid or the nautilus.

They usually have ocelli or simple eyes, although in some groups they may be more developed, for example in octopuses, which have quite advanced vision within molluscs.

Main types and groups of molluscs

The edge Mollusca It is usually studied in 7 groups and 8 classes:

  1. Gastropods (class gastropoda)
  2. bivalves (class Bivalvia)
  3. Cephalopods (class Cephalopoda)
  4. Scaphopods (class Scaphopoda)
  5. Polyplacophores (class Polyplacophora)
  6. Aplacophores (class Caudofoveata and class Solenogasters)
  7. Monoplacophores (class Monoplacophora)

Fossils of other extinct classes are also known, such as caraconchia (class Rostroconchia †) and the helcioneloids (class Helcionelloida †).

1. Gastropods

common garden snail
common garden snail (Helix aspersa)

Class gastropoda (from the Greek gaster«stomach, and podos"foot"), known as gastropods, gastropods or univalvesare the largest group of molluscs, representing up to 80% of all species of the phylum. They are also the most representative and similar to the model mollusk described above.

They are characterized by having a head or cephalic areaa ventral muscular foot very characteristic and dorsal shell, although in some types of gastropods the shell may be reduced. For example, land slugs they are gastropods without shell or with a small internal shell.

It is estimated that there are around 75 thousand species of existing gastropods, including animals as popular as the snails and slugsboth terrestrial and marine (not to be confused with sea cucumbers), or the limpets and abalone.

2. Bivalves

Clams on beach sand
Clams on beach sand

Class Bivalvia (bi-valva, “two valves or plates”) are a very numerous and popular group of molluscs throughout the world. Its most striking feature is the shell made up of two hinged valves through muscles and ligaments.

They are also known as pelecypodsclass Pelecypoda Y Bivalvia They are synonyms though. Bivalvia (Linnaeus, 1758) is much more widely used.

Among the bivalves we find marine molluscs, such as clams, oysters, mussels, razor shells or coquinas, and freshwater species.

In addition to the double articulated shell, the bivalves they lack a distinct head from the rest of the body. There are species that live buried in sandy soils (endofauna) or on them (epifauna), and also species attached to rocky surfaces.

3. Cephalopods

Common octopus (Octopus vulgaris)
common octopus (Octopus vulgaris)

Class Cephalopoda (Kephale"head", podos«foot») are marine molluscs characterized by a foot divided into several appendages or tentacles that start from the cephalic area. There is evidence of about 600 species, including the octopus, squid, cuttlefish and nautilus.

The nautiluses form their own subclass, the subclass Nautilinwhile the rest of the cephalopods are grouped in the subclass coleoid.

Nautiluses retain the outer shell and the foot is divided into numerous appendages called cirrus cloudssometimes they can have up to 100. Cirri are not tentacles proper, since they do not have suckers and are retractable.

In the rest of the coleoid cephalopods there is no shell, it is internal or it is replaced by internal cartilaginous plates. In addition, they have tentacles with suckers that generally appear in a fixed number; for example, octopuses (genus octopus) have eight tentacles.

4. Scaphopods

Scaphopod (Litoral de Granada – UGR)

Class Scaphopoda are a group of exclusively endofaunal shelled marine molluscs (they live under the substratum of the seabed). Its body is elongated and the mantle secretes a curved tubular shell similar to a fang.

5. Polyplacophore

Chiton cumingsii, the sea cockroach
Chiton or sea cockroach (Chiton cumingsii)

Class Polyplacophora (from the Greek polys"many", plax«plate», and phoros, "carrier") are small molluscs that live on rocky shorelines. Its body is elongated and flat, and is covered by a shell formed by overlapping several calcareous platesa characteristic that gives it its name.

The polyplacophores have a muscular foot of great strength compared to its size. The foot contracts on rocky surfaces and secretes an adhesive substance that helps keep the animal attached to the rock even in the presence of strong waves and water currents.

This behavior is also done by other quite similar molluscs, such as the limpets, but the limpets have a shell formed by a single piece and not by plates, so they are gastropods, not polyplacophores.

Polyplacophores are a small group compared to other mollusks. About 600 species are known and the most representative genus is the genus Hushbetter known as chitons, sea cockroaches or piragüeros.

6. Aplacophores

Falcidens spp.

Class aplacophora (from the Greek a"No", plakós«lamina» and phorus«carrier») are a group of molluscs that live in the deep seabed. In modern classifications it is usually divided into two, in the class Caudofoveata and the class Solenogasters.

About 300 living species are known, all of them without shell. They have a cylindrical worm-like body. Most species are very smallfrequently with a length of less than 5 cm, although there are some that exceed 30 cm.

7. Monoplacophores

Class Monoplacophora they are a group of single-shelled molluscs that may resemble limpets but are not related to gastropods. Currently, only species of the genus are known. neopilin.

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