Arthropod

Many animals have hard shells.

These shells cover their bodies. They help keep the animals safe.

The shells cannot stretch. So, the animals must shed them to grow. This is called moulting.

These animals have legs with joints. They also have bodies with many parts. Some are very small. Others are very large. There are millions of these kinds of animals.

Arthropods are a huge group of animals. They make up over 80 percent of all known animals. There may be up to ten million species!

Most have a hard outer shell called an exoskeleton. This shell is made of chitin, a tough material. Many shells also have calcium carbonate to make them strong.

Because the shell cannot stretch, they must grow by moulting. This is a way of shedding the old shell. A new, soft shell is revealed underneath.

Arthropods have bodies with many segments. They also have paired, jointed legs. These legs can be used for many tasks. Some legs act as claws for grasping. Others work as antennae to gather information. Many arthropods use compound eyes to see. Spiders use special eyes called ocelli to see images.

Some arthropods are very small. Others are very big. The Japanese spider crab can have a huge leg span. The American lobster can weigh over 20 kg.

Arthropods are a massive group of animals. They belong to a group called phylum Arthropoda. This group is the largest of all animal phyla. There may be up to ten million different species. They make up over 80 percent of all known animals. These creatures live in the air, on land, and in the water. They are very important to humans. They help grow our food by pollinating crops. However, some can also spread diseases to people and animals.

Most arthropods have a hard outer shell called an exoskeleton. This shell is made of a material called chitin. Many species also add calcium carbonate to make it even tougher.

The shell has three main layers. A thin, waxy outer layer helps keep moisture inside. The middle and inner layers are made of chitin and proteins. Because this shell cannot stretch, the animal must grow by shedding it. This way of growing is called moulting. The animal sheds its old shell, or exuviae, to reveal a new one.

Scientists have studied these animals for a long time. The word arthropod comes from Greek words meaning "jointed leg." A man named Barthélemy Charles Joseph Dumortier first used the name in 1832. Later, the German zoologist Johann Ludwig Christian Gravenhorst used the term "Arthropoda" in 1843.

We know these animals have been around for a very long time. Their ancestors date back to the Cambrian period. It is still hard for scientists to map out all their family trees. They are still actively debating how different groups are related.

Arthropods have many unique body parts. Their bodies are made of several segments. These segments can join together to form a head or a tail.

They have paired, jointed legs that work like specialized tools. Some legs act as claws to grab things. Other legs might work as gills to help them breathe underwater. Many use compound eyes to see the world. Spiders are different because they use special eyes called ocelli to form images.

There is a huge range of sizes in this group. Some arthropods are tiny and hard to see. The smallest ones belong to a group called Tantulocarida. On the other side, some are very large. The Japanese spider crab has legs that span a huge distance. The American lobster can weigh over 20 kg.

Whether they are small bugs or huge crabs, they are all part of this amazing family.

Arthropods are invertebrates belonging to the phylum Arthropoda. This group is the largest animal phylum on Earth. Scientists estimate there are between 1.17 million and 10 million different species. This means arthropods account for over 80 percent of all known living animals. They are vital to our world because they act as pollinators for many crops. However, some species can also spread diseases to humans and livestock. They are among the few animal groups to successfully adapt to dry land.

The defining feature of an arthropod is its exoskeleton. This is a hard outer shell made of a polymer called chitin. The cuticle, or outer layer, usually consists of three distinct parts. First is the epicuticle, a thin waxy coat that prevents moisture loss. Below that is the exocuticle, which contains chitin and hardened proteins. The innermost layer is the endocuticle, made of chitin and unhardened proteins.

In many crustaceans and beetles, the shell is biomineralized with calcium carbonate. This makes the armor even tougher and stronger for defense.

Because an exoskeleton is rigid, it cannot stretch as the animal grows. To increase in size, arthropods must undergo ecdysis, also known as moulting. During this process, the animal sheds its old shell, called an exuviae. A new, softer exoskeleton is revealed underneath. The animal then grows until it is time to moult again.

This cycle continues until the arthropod reaches full adult size.

Arthropod bodies are built from repeated segments. These segments can group together into specialized regions called tagmata. For example, many insects have a body divided into three parts. Spiders have a different two-part body structure.

Most arthropods have paired, jointed appendages. These limbs are highly specialized, acting like a Swiss Army knife. Some appendages function as gills for breathing, while others serve as claws, antennae, or walking legs. In some species, these limbs have even vanished entirely.

Sensory systems in arthropods are incredibly diverse. Most use a combination of compound eyes and pigment-pit ocelli. While compound eyes provide most visual information, ocelli often detect light direction. Spiders are a notable exception. Their main eyes are ocelli that can actually form images. Some spider eyes can even swivel to track moving prey.

They also use setae, which are tiny bristles projecting through the cuticle. These setae act as mechanical or chemical sensors to detect touch or currents.

Internal systems also show great variety. Most arthropods have an open circulatory system. Instead of veins, they have a body cavity called a haemocoel. A fluid called haemolymph circulates through this cavity to reach internal organs. Their nervous systems are ladder-like. They feature paired ventral nerve cords that run through every segment. These cords form clusters of nerve cells called ganglia in each segment. In the head, these ganglia fuse to form the brain, which encircles the esophagus.

The history of arthropods stretches back to the Cambrian period. Scientists study fossils like Marrella to understand their early ancestors.

The name "arthropod" comes from the Greek words for "jointed leg." The term was first used in anatomical descriptions by Barthélemy Charles Joseph Dumortier in 1832. In 1843, the German zoologist Johann Ludwig Christian Gravenhorst first used the designation "Arthropoda." Determining exactly how all these different body types evolved is known as "The arthropod head problem."

There is an enormous scale of size within this phylum. The smallest arthropods are crustaceans in the class Tantulocarida, some measuring less than 1 mm. In contrast, the largest are found in the class Malacostraca. The Japanese spider crab has legs that can span up to 3.7 meters. The American lobster can reach weights exceeding 20 kg. Whether they are microscopic or massive, arthropods remain a dominant force in every ecosystem on Earth.