Characteristics Of Phylum Mollusca With Examples

Phylum Mollusca is the second most common phylum and they play a huge role in the lives of humans. As a great source of food, pearls and jewelry, they are fascinating creatures.

In the animal kingdom, Phylum Mollusca is made up of soft body animals which usually have a shell. This article will walk you through the Phylum Mollusca and, before you know it, you’ll be an expert on the topic!

If you’re looking for information on the characteristics of Phylum Mollusca, as well as some examples of the specific types, you’ve come to the right place.

And I don’t blame you for your interest in these fascinating organisms! They truly are fantastic things to study and understand.

What Is Phylum Mollusca?

A Phylum Mollusca is a type of mollusc, the study of which is known as Malacology. Molluscs are most commonly found in freshwater.

Phylum Mollusca are soft-bodied animals. This means that they are animals that do not have skeletons. They are made up, instead of muscles that allow them to move.

These animals have a muscular foot and a distinct head. The Phylum Mollusca is the second most common type of phylum in the world.

History Of The Phylum Mollusca

Mollusca is a name that comes from the Latin word ‘mollis’. This means ‘soft’ and was first used in 1798 to describe certain marine creatures.

It was originally used to describe squids and cuttlefish. These animals either have shells that are internal, or they don’t have shells at all.

Later on it became accepted that these species and other molluscs were linked together. Snails were later categorized as molluscs.

In Malacology – the study of molluscs – there have been over 160,000 species identified. Of this, 128,000 are currently living and the remaining 35,000 have been identified as fossils.

Characteristics Of Phylum Mollusca

Characteristics Summarized

The Phylum Mollusca has many different characteristics.

• They mostly are freshwater and marine animals so they are aquatic. However, they do have a few terrestrial forms.
• They can be of varied sizes from small clams to the larger squid.
• They are soft-bodied and bilaterally symmetrical.
• They are usually composed of a head, a soft body and visceral masses. Some have muscular feet.
• They have a single-layered epidermis which covers over the body.
• They often have a shell which protects the main part of the body. This is the part of the body that contains the organs.
• The mollusks have a head and some have eyes, mouth and tentacles. Some have some of these but not others.
• Some classes have a foot which is used for movement. They can either crawl, swim or burrow.
• The visceral mass is where the body’s vital organs are.
• They have a simple digestive system. They have a mouth and an andus. The intestine varies from one to the other.
• In most, the circulatory system is open.
• Many of them breathe through gills.
• They either breathe through direct or indirect respiration. They breathe through the gills or the lungs.
• Some excrete through the kidneys or by paired metanephridia.
• The nervous system varies but is usually made up of cerebral and pleural and pedal ganglia pairs.
• They have touch, smell and sight senses through their eyes, statocysts and receptors.
• They are mostly dioecious (meaning they have both male and female), but some are hermaphroditic (meaning they can reproduce on their own).
• External or internal fertilization.
• Development in reproduction occurs either directly or through a stage of metamorphosis which occurs on the sea bed.

Soft-bodied animals are creatures with a soft body who are enclosed in a shell. Phylum Mollusca has many different characteristics and these are listed below:

Habitat

Phylum Mollusca contains many different animals that live all over the world. They are mostly found in either freshwater or marine habitats.

There are a few of these species that live in terrestrial environments, there are the only mollusks who don’t respire through lungs, but instead breathe through lungs.

In freshwater habitats, freshwater mollusks are found in things such as streams or lakes. Some other creatures also fall into the Phylum Mollusca category such as snails and mussels.

Most Phylum Mollusca are found in marine environments, from shallow to deep water.

For example, Chitons are found along the coastline in the shallower water, while caudofoveata are often found in deeper water on the seabed. The mollusks that live in the water usually breathe using gills.

Certain types of Phylum Mollusca can be found across all three of the different environments. Gastropods fall into this category, and there are 65.000 species of Gastropods!

Which species are found in specific habitats is impacted by temperature. Some mollusks can live well in extreme temperatures.

Warmer temperatures encourage growth in certain animals, such as snails and the temperature also increases reproduction in these animals.

The type of environments that mollusks reside in depends on their specific needs. Those who reside in freshwater environments are residing in habitats with high calcium levels.

This is believed to be because these high levels of calcium help with shell construction.

The many different types of mollusks are very suitably adapted for certain habitats. These are some adaptations that the mollusks have made that are believed to be linked to their lifestyle and habitat:

Freely Swimming Mollusks

Certain mollusks can swim. The mollusks that can swim are members of the Cephalopoda species, such as squid. These mollusks swim using fins. Not many mollusks have fins but squid and cuttlefish do.

It is not just the fins that allow them to swim, it is also the tentacles and a small jet that they have that allow them to be stable.

Burrowing Mollusks

These mollusks are called burrowing mollusks for a very specific reason. They make burrows in soft sediments.

To make these burrows they have a large foot and shell hinge teeth that allow them to burrow through soft sediments. Some examples of these mollusks are gapers, cockles and peppery furrow shells.

These mollusks burrow in such an interesting way. They use their feet to push into the sand and make it soft enough for them to push through it.

Then, blood is pushed into the foot which anchors the mollusk into the floor. This anchor means that the shell can be rocked into the sand, starting the burrow.

During this stage, the mollusks release little bits of water around the shell which makes it easier for them to burrow down into the ground.

Crawling Mollusks

Crawling mollusks are found in all three of the habitats listed above. Marine, freshwater and terrestrial.

These are the most well known mollusks that we see on a regular basis. Snails and slugs are crawling mollusks. They move using their muscular foot which contracts and relaxes to move the mollusk forward.

They release slime as their foot contracts and relaxes, giving them a layer to move over and gives a direction to their movement.

This is how snails and slugs move on ground, and other mollusks use the same method to crawl in marine and freshwater environments.

Anatomy

Most members of the Phylum Mollusca are symmetrical organisms. They are usually made up of a visceral mass, a muscular foot and a head.

There are many different species and they range quite a lot in their form. They differ greatly in size and structure.

Groups Of Phylum Mollusca

The 4 most common groups of Phylum Mollusca are Gastropoda, Bivalvia, Cephalopoda and Polyplacophora.

Gastropoda

Gastropoda are a class that contain gastropods which include snails and slugs. These species have the following anatomy.

The heads of these species are very specific. They have distinct heads with tentacles on the top of their head, and eyes at the end. These species also have rows of teeth in their mouths.

They also have a visceral mass. This is the main body of the organism and it is just underneath the head. It houses all the important organs such as the lung, the liver and the digestive system.

It also includes the reproductive organs. Some organisms have a shell that encloses the visceral mass such as snails. Slugs don’t have this. These animals also have a muscular foot. This is usually flat and quite broad.

Bivalvia

Some examples of the Bivalvia class of Phylum Mollusca are muscles, oysters and clams. The heads of this class are very distinct as their heads are very underdeveloped. The body section of the Bivalvia is covered by two shells.

The hinge of these shells consist of some teeth. These two valves contain a material which is released inside the two shells which allows the shell to grow.

The body of these mollusks contain a muscular foot and gills. They also contain the heart of the mollusk and the digestive system.

Cephalopoda

Squid and cuttlefish are part of the cephalopoda class of mollusk. The heads of the creatures are well-developed, unlike the Bivalvia.

They have a jaw which looks slightly like the beak of a parrot which allows them to bite their food. This class of mollusk has been said to have the most complex brains of any invertebrate, according to research.

The head is in between the tentacles and the visceral mass. The tentacles are a modified foot. The foot is modified in this class to look like tentacles that are on the top of the head. These help them to catch their food.

Polyplacophora

This class contains chitons which are similar to the organisms of a snail. These mollusks have a head and they have a very obvious head with many teeth. The flattened body is covered with a shell which is behind the head.

The mantle which is underneath the shell contains all the organs, including the heart, intestine and stomach as well as the organs that aid reproduction.

Reproduction

One key characteristic of any organism is the way in which they reproduce. The majority of mollusks are dioecious. This means that they have male and female individuals. The fertilization occurs internally.

They are also oviparous, which means that the development of the mollusks are either direct or indirect. Some mollusks do have the ability to self-fertilize, these are known as hermaphrodites.

Snails that live in freshwater are a key example of a hermaphrodite mollusk. When these snails reproduce, they produce both sperm and eggs and reproduce in this way.

Proteins and nutrients get added to the egg which then passes through the hermaphroditic duct and is fertilized by the sperm.

In these hermaphroditic snails, they still possess the ability to be fertilized by other snails. This is a preferred way of reproduction.

Some of the phylum organisms are able to change their sex several times during their lifecycle.

In dioecious species of Phylum Mollusca, the separate female and male individuals work together to reproduce. The different species have certain organs that they use as their reproductive organs.

For instance, in viviparidae, their right tentacle is the organ that releases the sperm, while in hydrobiidae, they have a specialized penis which releases this sperm. Females will release their eggs which will meet the sperm cells and reproduce.

In most creatures, the fertilized cells will settle on the sea bed or at the bottom of the water where they will undergo metamorphosis and become adult forms.

This is not true of all mollusks as some reproduce by directly developing and don’t require this stage. A clam is an example of a species that undergoes direct development.

Eating And Nutrition

Because species attached to the Phylum Mollusca are found in various habitats, their food is also very varied.

Snails and slugs rely on animal waste and plants as well as various other things that they come across in their path. They will eat whatever they find.

They have thousands of teeth in their mouths which are microscopic, meaning they can easily adapt to eat whatever they come across.

Another type of eating pattern for mollusks is filter feeding. This is mostly seen in bivalves and they feed on different matter in their surroundings.

This works through water passing over their gills which allows food particles to become trapped in the gills which the mollusks then eat.

Some larger species use their tentacles to capture their prey, for example crayfish and other mollusks. This is specifically for the mollusks who are quite active.

Systems

Circulatory System

While humans have a circulatory system which runs through blood vessels, mollusks have a system that is more open, using the main part of the body as a cavity for the blood to travel around the body.

The blood travels fluidly between the body and the tissues. Blood enters the blood vessels of the organism and is transported to the gills. This is where oxygen is absorbed and carbon dioxide is given out.

Oxygenated blood is then taken to the heart. It is then pumped around the body of the organism again.

Brain And Nervous System

Most mollusks don’t have a brain, however certain types do. Cephalopods do have a brain.

Mollusks have a nervous system though, which is a collection of nerves that are all in certain areas of the organism. This area is the ganglia. These are located in the main organs, as well as the head and the mouth.

Classification Of Mollusks

Mollusks have been categorized into six different classes. These are listed below:

• Monoplacophora: These are usually symmetrical and have a dome-shape.They live in marine habitats and they have five pairs of gills. The heart of this class has two pairs of auricles and a single ventricle.
• Amphineura: These live in marine habitats and have a long body and a small head.
• Scaphopoda: These are also marine animals and they have a bilaterally symmetrical body. They are long and their main body is enclosed in a shell. They have a mouth with tentacles. These mouths are surrounded by lobular processes. They have dioecious sexes.
• Gastropoda: These are the biggest class and they are mostly marine. Some are freshwater and some are terrestrial. Some have shells and some don’t. They have a developed head which has eyes and tentacles to catch prey. They have a large foot and a detailed digestive system. They breathe through their gills and they have separate sexes.
• Pelecypoda: These mollusks live in marine and freshwater environments. They have gills and their sexes can be either separate or united. They get their food through filter feeding.
• Cephalopoda: These mollusks live in marine habitats and they have a long body and are symmetrical. They have developed eyes and a mouth.

Economic Importance

The Phylum Mollusca holds a large amount of economic importance. They are used as food across the world. Oysters, cuttlefish and squid are all very popular foods in many different countries.

The shells of certain molluscs also have a lot of value as they are used in the process of pearl extraction. Medicinal value is given to sepia ink.

As well as this, the shells that belong to the oyster help with making roads as they are mixed with tar.

Snails also hold a great amount of importance. They contain lots of antioxidants and vitamins such as iron and calcium. They are also believed to hold properties that prevent or fight cancer.

History Of Evolution

Modern mollusks are so varied in their anatomy compared to each other. They also vary greatly in terms of their behavior.

Looking at the evolutionary history of these mollusks is very difficult because of this, as it is very hard to work out.

Naturalists have researched it at length and they believe in a hypothetical ancestral mollusk which displays the characteristics of most of the mollusks that we see today.

They had a muscular foot, tentacles, shells and a lot of other features of the various mollusks we see today. It seems that they date back a very long time.

Frequently Asked Questions

What Is A Soft-Bodied Organism?

Soft-bodied organisms lack a skeleton. They can move using their muscles but they have not developed the bones that are known as a skeleton.

What Are Some Examples Of A Phylum Mollusca?

There are many different types of Phylum Mollusca. An octopus is a great example of a Phylum. Squids, oysters and cuttlefish are also great examples of them and they are used as food in many countries around the world!

There are over 160,000 species of the Phylum Mollusca identified, 128,000 of which are currently alive. The other remaining species have been identified through fossils.

How Many Gills Do Mollusks Have?

Different classes of mollusk have different numbers of gills. This varies greatly. Most have only one pair, or even just one gill. Some have up to 5 gills. These gills are used for filter feeding as well as breathing.

The mollusks will eat lots of sediment from the water by filtering it through their gills. In terms of breathing, these gills filter oxygen from the water to allow the organism to breathe.

Do Mollusks Have A Brain?

Some mollusks have collections of neurons in various places across the body rather than having true brains.

Some of the more developed mollusks have brains which are more developed rather than just the collections of neurons. These clusters are called ganglions.

Do Mollusks Have A Tongue?

Some mollusks do have a tongue and it is used to help them to eat. Some have mouths with microscopic teeth. The teeth are on the tongue inside the mouth and they allow the mollusks to have a very varied diet.

They eat various different foods, some meat and some plants so they have to have the ability to eat and digest these foods. They have a relatively simple digestive system.

Final Thoughts

Now you have all the information you need on Phylum Mollusca and you can impress everyone with your knowledge!

You’re now an expert on the topic and you can recite all of the characteristics of these mollusks, as well as list off some examples of the different types and classes.

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