Animals with Exoskeleton - Names and Examples

The exoskeleton is an external structure which covers the body of various animals, including arthropods. It is a rigid structure, resistant to trauma and damage from outside forces. However, as with all animal development, there are different stages of the exoskeleton, so its consistency will depend on the time of its life cycle.

Another variable aspect of exoskeleton animals is the material from which they are made. The structure is created from various materials depending on the subphylum (the taxonomic category) to which the animal belongs. For example, for insects and fungi the exoskeleton will be made from chitin, a polymer also used to create squid beaks and fish scales. The exoskeleton of molluscs (their ‘shell’) is predominately made of calcium and silica for diatoms.

Exoskeletons can take many forms, but there are some general parts which create their structure. These include:

• Cuticle: this is the non-cellular layer secreted by the hypodermis. Arthropods are able to shed this layer to facilitate growth.
• Hypodermis: the cellular structure that generates the cuticle and creates the necessary conditions for its moulting.
• Basal membrane: innermost non-cellular layer of the exoskeleton, is responsible for supporting the muscles.

What is the function of an exoskeleton?

Just like an endoskeleton, an exoskeleton has various purposes. These include:

• Protection and support: exoskeletons give the animal form so they can carry out their daily activities. They support appendages such as legs so that the animal can move from one place to another. They also protect vulnerable internal organs, muscles and other tissue.
• Sensing: the exoskeleton can help the organism to be aware of external agents, such as moisture and dryness.
• Storage: they can store certain substances and nutrients which can be used for various purposes. These substances often create colors whch can be used for predation or defense.
• Feeding: the exoskeleton allows the animal to use its structures for feeding, as well as excreting.

In a minority of species, it is possible to have an endoskeleton as well as an exoskeleton. However, there is some debate over true exoskeletons which we will explain later.

The phylum of arthropods contains the largest number of species in the world. All of them have exoskeletons, but what are example of some of these exoskeletal animals? To better understand such a large group of animals, we can break down the classification of different arthropods:

Chelicerata

One of the major subdivisions of arthropods, they have segmented bodies with jointed limbs attached. Their exoskeleton is made of chitin and proteins. They can be either terrestrial or aquatic, Included under this umbrella are:

• Arachnids: includes spiders, ticks, mites, scorpions and many others (a lot of which are extinct). In total, it includes more than 30,000 different species.
• Pycnogonida: includes 50 species of sea spiders, which usually inhabit the body of other animals.

Euarthropoda

It includes the Mandibulata superclass, a group of terrestrial arthropods which are classified by the fact they have jaws. They also have well-formed antennae and legs. They cover the following classes:

• Insects: they have a well-defined head, thorax and abdomen. They live in both terrestrial or underwater environments, many of which have wings. There is great diversity among insect species with carious characteristics and particularities.
• Crustaceans: have a rigid exoskeleton due to calcareous substances. They live in fresh or salt water and include a great variety of species with multiple appearances, from lobsters and crabs to woodlice.
• Myriapods: they develop an elongated body that makes them similar to worms, but with a differentiated head and the addition of legs.

In turn, the subclass of the Myriapoda is divided into:

• Diplopoda: this class includes millipedes. They have jaws, an elongated body and many legs.
• Chilopoda: similar to the diplopods, this is the class which contains centipedes. They have a more elongated body than millipedes.
• Symphyla: elongated species similar to the chilopoda, but smaller and with antennae.
• Pauropoda: these are terrestrial worms that like to live in the soil. They have antennae and tracheal breathing.

Now that you know more about exoskeletons as external structures, we can look more closely at exoskeleton animals:

1. Mites

From subclass Acari, it is parasitic arachnids living off of other animals. There are terrestrial and aquatic species, and their food is varied: hematophagous (meaning they live off of blood), detritivores (sustain themselves on body parts, skin flakes and other detritus from the host) and herbivores (plant matter). The different species are capable of transmitting diseases.

2. Crabs

There are many types of crab, but true crabs are crustaceans with five pairs of legs. They have a body with a strong shell and claws which allow them to defend themselves as well as catch their prey. Not many can swim, so they mainly live on the sea bed, although there are land crabs also.

3. Bees

Bees are insects that are widely distributed on planet Earth. They are organized in colonies with well-defined hierarchies and are very important in the process of pollination for thousands of plant species. Without them, we humans may likely die.

4. Centipede

Under the banner of Centipede, there are more than 3,000 species of arthropods characterized by an elongated body divided into segments, multiple legs, antennae and powerful jaws. They are predators and hunt insects, worms and snails.

5. Marine spiders

They have eight legs that they use to move, which is why they are compared to terrestrial spiders. They mimic the seabed easily for camouflage and have a body of thin and long limbs.

The endoskeleton is an internal structure which protects the organs, allows the muscles to be strengthened, provides shape to the body of the animal, allows movement and protects the nervous system. All vertebrates have an endoskeleton, so the most common animals have these structures.

A difference between the exoskeleton and the endoskeleton is that the latter grows with the individual to which it belongs. It is not necessary to discard the skeleton to perform this function. In addition, it develops from the moment at which the organism is an embryo (fertilization). Endoskeletons are characterized by a vertebrae (or spine) made up of discs, which protect the nervous system and connect with the skull and brain in most species.

Exoskeletons do not grow along with the animal. For this reason, they will need to be shed. When the animal needs to grow or even metamorphose into a different form (e.g. caterpillar to butterfly), they will shed the exoskeleton and emerge anew. The new exoskeleton will usually be much softer and will take some time to harden.

As we stated earlier, there are some animal species which are thought to have both exoskeletons and endoskeletons. However, this is not the case. In actuality, they have an endoskeleton which exhibits in a strange way. These animals include:

1. Armadillos

Armadillos are terrestrial mammals and, therefore, have an endoskeleton inside their body. However, these animals are also characterized by having an outer shell formed of bony plates, which serves as a means of protection. This shell actually has living tissue on the outside and is not considered a true exoskeleton.

2. Pangolins

Pangolins are mammals that are distributed in Asia and Africa, where they feed on ants and termites. As for its appearance, it is similar to armadillos since, in addition to being a vertebrate, it has an external armor formed by rigid plates. These plates are made of keratin and are not an exoskeleton, despite being very hard and being very protective. Human and animal hair is also made from keratin, yet is not actually part of the skeleton.

3. Turtles

Marine and freshwater turtles have an exoskeleton that is very recognizable: the carapace. The shell protects the organs and, at the same time, represents a kind of "house" for the turtle, where it can take refuge to rest or avoid predators. It is not, however, actually a skeleton.

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