Mollusks: characteristics, types and examples

Mollusks are characterized by being triblastic, coelomate, protostomate and have, at least initially, bilateral symmetry. This group has both aquatic (marine and freshwater) and terrestrial representatives. In general, mollusks have a body divided into three zones: the head, the foot and the visceral mass. Its dorsal wall forms a pair of folds that fall on both sides of the body and constitute the mantle, which has a protective function and defines a space known as the paleal cavity, where the gills or lungs of the mollusk are housed. There is a series of functional and structural elements common to the entire edge, although some representatives may present them more or less modified. These are the main characteristics of mollusks :

The foot of the mollusks

It is a muscular structure that can serve for locomotion or be reduced and serve to anchor the body to the substrate, as in the case of bivalves.

Shell

Calcareous structure secreted by the mantle that has a protective function. It can be very varied and present twists, shells, numerous pieces, be outside, which is the most common, but also inside the body or even have disappeared, as in octopuses and sea slugs.

Gills in mollusks

They can have respiratory function and also serve for nutrition. They are normally found arranged in rows in the cavity of the mantle, which connects with the external environment. The most typical gills are bipectinate, which have an axis and filaments arranged on both sides of it, although there are also monopectinate gills, which have filaments only on one side.

As a curiosity, in this other post you can discover +40 animals that breathe through gills , some of them being mollusks but also fish and amphibians.

Radula

It is a scraping organ that is found in the anterior part of the digestive tract, next to the oral cavity. It is composed of a ribbon-like membrane covered with small backward curved chitin teeth that is supported by a cartilaginous structure called the odontophore. When removing the radula, the teeth slide on the surface of the food and, when retracting it inwards, these dig into the food and carry it towards the mouth.

Style

It is a tubular and elongated structure of rigid consistency, located in a sac with ciliated walls. The movement of the cilia causes the style to rotate against a part of the stomach that is covered by a chitinous plate and, in this way, works like a mill, thus crushing the nutrients.

The circulatory and nervous systems of mollusks

The circulatory system of these animals is open and is made up of blood vessels and sinuses without their own lining. The oxygenated blood from the gills enters the heart through 1 or 2 atria, passes into a ventricle, and exits through an aorta to the blood sinuses of the different parts of the body. From the visceral mass, the deoxygenated blood passes through the kidney and returns to the gills.

The nervous system of mollusks is formed by a nervous ring that surrounds the esophagus, from which a pair of nerve cords that go to the foot and another pair of nerve cords that go to the visceral mass. In addition to the osphrads, which serve to control the particles that enter the mantle cavity with water, they have other sensory organs such as tentacles, eyes, rhinophores, photoreceptors, and statocysts.

Reproduction of mollusks

Regarding reproduction, there are more primitive species that are dioecious (of separate sexes), although, generally, mollusks have 2 gonads next to the coelom in the visceral mass. When the mollusk reproduces, the gametes pass, through the nephridial duct, into the cavity of the mantle and, from there, outside with the exhaling current. Fertilization is external and the embryo turns into a typical larva called “trochophore larva”, which have the shape of a spinning top, although with evolution, this larval form has been replaced by another called “veligeous larva”, which has a veil that it serves to swim.

Excretory system

Mollusks have a metanephridial excretory system that collects 2 filtrates from metabolism. The metanephridiums are excretory organs that are responsible for collecting the filtrate through a part called the nephrostoma and leading it through the nephroduct to the cavity of the mantle. As it passes through the excretory tubule, the urine is modified by the reabsorption of the material until, finally, it reaches the nephridiopore.

The current systematic recognizes 7 classes of mollusks: polyplacophores, monoplacophores, aplacophores (which include caudofoveados and solenogastros; recognized as two independent classes depending on the bibliography consulted), scaphopods, pelecipods, gastropods and cephalopods. Thus, these are the 7 types of mollusks and their characteristics :

Gastropods

They constitute the largest group of mollusks , with some 35,000 living species and some 15,000 fossils. It has representatives dating from the Cambrian. Since then, primitive gastropods have given rise to very diverse morphological and functional designs, occupying different habitats, both marine, freshwater and terrestrial. They present varied trophic customs: carnivorous, herbivorous, parasitic, saprophytic …

Many gastropods have a coiled shell, which is formed from an area called the apex. The shells are important for the taxonomic classification of the species and are made up of several layers: an outermost part called the periostrac, composed of protein and tanned with quinone, and a mineral part below the periostrach formed by several layers of calcium carbonate in the form of calcite or aragonite.

Gastropods are normally divided into 3 groups: prosobranchs, opisthobranchs, and lungs. Prosobranchs include the most primitive gastropods, such as abalone (genus Haliotis ) or limpets. They have a flattened shell and, due to their diminished body thickness, the mantle cavity is sometimes displaced to the left, while the visceral mass is located to the right. Opisthobranchs tend to shrink and lose the shell, shrink the mantle cavity, and acquire secondary bilateral symmetry. This group includes bubble snails (eg Hydatina and Acteon ), sea ​​hares, nudibranchs, etc. Finally, pulmonary gastropods are characterized because many are terrestrial, (although there are also freshwater fish) and because, to form a lung, the edges of the mantle cavity have fused and there is only an opening that connects the lung with the outside called “Pneumostoma”. There are representatives with a shell (such as H e lix ) and others without it, which are called “land slugs”.

Bivalves (or pelecipods)

They have more than 9,000 species and owe their name to the fact that their shell is made up of two articulated valves that protect the individual. The body of bivalves is laterally compressed and their foot is reduced because it is not used for locomotion, but is used to excavate and hold on to the substrate. The shell valves are composed of the same parts as in gastropods (periostracus and mineral part) and, under these, the mantle is arranged, which has three folds: the internal, which contains the musculature, the intermediate, which is of sensory character and can present tentacles, eyes and chemoreceptor organs, and the external fold, which is the one that the shell secretes primarily.

The bivalves are differentiated into two large groups: the protobranchs and the lamellibranchs. Protobranchs are more primitive bivalves, have bipectinate gills and feed on bottom debris. Lamellibranchs are filter feeders with evolved gills attached to the walls of the mantle cavity and forming vertex structures called “alimentary grooves.”

Cephalopods (or siphonopods)

It is a very ancient group of mollusks of which about 7,500 fossil species and about 800 living species are known. They are typically pelagic organisms, although most have adopted a benthic way of life (associated with the seabed). Squid, octopus, cuttlefish, and nautilus are included in this group.. The body of cephalopods has been elongated in a dorso-ventral direction and they have a mouth surrounded by a variable number of tentacles. The most current species have a reduced shell (as occurs in cuttlefish and squid) or, directly, non-existent (as in octopuses). Only nautiluses have a clearly developed shell. In cephalopods, the movement of water in the mantle cavity serves both for the animal’s locomotion and for gas exchange. Depending on the species, they have a different number of gills, with specimens that have evolved to completely reduce their gills, so that they breathe through the surface of the body.

Cephalopods have a more complex and evolved circulatory system than that of other mollusks because it is a closed system and consists exclusively of vessels that are lined with endothelium. In turn, they present an ischemic heart, from where the blood leaves the ventricle through an anterior and a posterior aorta. Cephalopod blood contains hemocyanin.

In addition, cephalopods have a highly evolved nervous system that has undergone a concentration of ganglia in a typical brain, from where the motor system is controlled to coordinate the movement of the individual. Among the most prominent sensory organs of cephalopods are the eyes, which are highly evolved.

Monocophores

They are small marine mollusks that have existed since the Cambrian, and are currently only represented by 2 genera – V ema and Neopilina – and 8 species. They have a unique shield-shaped shell below which there is a repetition of morphological elements (gills, nephridia, atria, retractor muscles …) throughout the body.

Polyplacophores

Group of mollusks that has about 500 species and has a shell made up of a series of ceramas interwoven, which gives them a certain capacity for body articulation. Their body shape is adapted to hold onto the substrate, for which they use a foot that protrudes below the shell. The cavity of the mantle where the gills are forms a kind of closed channel that runs longitudinally through the body and communicates with the outside through two anterior holes (where the water enters the cavity) and two posterior holes (where it leaves).

Aplacophores

Group that includes about 180 species and owes its name to its lack of shell. There are two groups: solenogastres and caudofoveados . The solenogasters are elongated and, in addition to the shell, they also lack a mantle and foot cavity, and are characterized by having a groove on the ventral surface of the body that runs longitudinally through the individual. Caudofoveados are burrowing mollusks that can reach 10 mm in length and that live in sediment. They have a cylindrical body with a chitinous cuticle and covered by calcareous spicules.

Scaphopods

They present 350 species and are characterized by having a fang-like shell. They inhabit the sandy seabed (from 6 meters deep), arranged upside down. With their feet, they dig into the sediment, from where they get their food and position themselves with the widest part of the shell facing the sediment, while the narrowest part has a hole that faces outwards so that water can enter and exit. the mantle cavity, along with the waste material. They lack gills, eyes, and osphrads.

To conclude this summary on mollusks that includes their characteristics, types and examples, some common types within this phylum are mentioned below.

• Sea hare: like Aplysia , characterized by having the inner shell.
• Nudibranchs: sea slugs, lacking mantle cavity, shell and original gills (some specimens have external ones).
• Nautiluses (genus Nautilus ).
• Octopus (Octopoda).
• Cuttlefish (Sepiida).
• Common garden snail ( Helix aspersa ).

To learn more about this large group of animals that are mollusks and others related to them, we recommend this other article on Invertebrate animals: examples and characteristics .