Malacostraca

Many animals belong to this big family.

They live in the sea.

Some live in fresh water too. Some even live on land.

Crabs and lobsters are in this group. Shrimp and krill are here too.

Most of them have hard shells. They have many body parts. They also have legs with joints.

Some have eyes on stalks. They can move their eyes around. It is a very big group of animals.

There are about 40,000 kinds of malacostracans living today.

This is a very big group. It is the second largest group of pancrustaceans. Insects are actually the largest group.

You might know some of these animals. They include crabs, lobsters, and shrimp.

Krill and prawns are also part of this group. Some live in the ocean. Others live in fresh water or on land.

Most malacostracans have a body with 20 parts. These parts form a head, a thorax, and an abdomen. The head has two pairs of antennae. Many have eyes on moveable stalks. This helps them see better.

Some animals, like the mantis shrimp, are hunters.

They have strong claws to catch food. Most others are scavengers. This means they eat bits of food they find. They use a special stomach to grind up their meals. This part works like a gizzard. It has hard ridges to crush food.

Malacostraca is a huge group of animals. It is the second largest class of pancrustaceans. Only insects are larger than this group. There are about 40,000 living species today. This group includes many familiar creatures. You might know crabs, lobsters, or shrimp. It also includes crayfish, prawns, and krill.

Some members are less common, like isopods or amphipods. They live in the ocean and fresh water. Some even live on land.

Most of these animals follow a specific body plan. They have 20 body segments. These segments form a head, a thorax, and an abdomen. The head has two pairs of antennae. Some antennae have branches. Many species have compound eyes on moveable stalks.

This helps them see their surroundings. Their mouthparts include mandibles and maxillae. A hard shell called a carapace may cover them. In many, the thorax and head are joined together. This joined part is called a cephalothorax.

Scientists have studied these animals for a long time. The name Malacostraca comes from Greek words. Aristotle used this name a long time ago. He compared them to oysters. He noticed their shells were more pliable or soft. A French scientist named Pierre André Latreille used the name in 1802. He worked at a museum in Paris. He was a curator for arthropods. The name is actually a bit of a mistake. The shells are only soft right after moulting. Usually, the shells are quite hard.

These animals have many different ways of living. Most are scavengers that find bits of food. Some are carnivores that hunt other animals. The mantis shrimp is a great example of a hunter.

They have powerful claws to catch prey. Inside, they have a special stomach. It has a gastric mill to grind food. This part uses hard ridges and teeth. They also have an open circulatory system. A heart pumps blood through a body cavity. This blood carries oxygen and nutrients.

Malacostracans are linked to many things you see every day. You might see a woodlouse in your garden. These are isopods.

You might also see a hermit crab on a beach. These are also part of this group. Even the tiny krill in the sea are malacostracans. They all share similar body parts and ways of growing. They have lived on Earth since the Cambrian period. This makes them an ancient and successful group of life.

Malacostraca is a massive class of pancrustaceans. It is the second largest class in this group, following only insects. Today, there are approximately 40,000 living species within this class. They are divided into 16 different orders. This group is incredibly diverse in body form. It likely contains more variety in shapes than any other animal class.

Members include crabs, lobsters, shrimp, and krill. It also includes less familiar animals like isopods and amphipods.

Most malacostracans follow a specific body plan. They are segmented animals with 20 body segments. These segments are organized into three main sections called tagmata. These include a five-segmented head, an eight-segmented thorax, and a six-segmented abdomen. The abdomen ends with a structure called a telson. A few primitive members, like the Leptostraca, have seven abdominal segments instead.

Each segment usually bears a pair of jointed appendages. These appendages help the animal move, eat, or breathe.

The head and thorax have many specialized parts. The head features two pairs of antennae. The second pair often has branches called exopods. These can flatten into scales known as scaphocerites. Mouthparts include mandibles, maxillules, and maxillae. Most malacostracans have compound eyes. In many species, these eyes sit on moveable stalks.

Some species may have unstalked eyes, or lose them entirely. In many cases, the head and thorax fuse into a cephalothorax. This structure is often protected by a hard shell called a carapace.

Internal systems allow these animals to thrive in many environments. Their digestive tract is straight and includes a two-chambered stomach. Inside the stomach is a gastric mill. This part uses chitinous ridges and teeth to grind food. They also have an open circulatory system. A heart pumps blood into a body cavity called a hemocoel. This blood carries oxygen and nutrients to the organs.

The respiratory pigment used to carry oxygen is called haemocyanin. They also have sensory organs like statocysts and sensory bristles.

History shows that malacostracans are an ancient group. The first ones appeared during the Cambrian period. The name Malacostraca has an interesting history. Aristotle used the term to contrast them with oysters. He noted their shells were more pliable. In 1802, Pierre André Latreille applied the name to this taxon. He was a curator in Paris. The name is actually a misnomer. Their shells are only soft immediately after moulting. Most of the time, the shells are quite hard.

Scientists divide the class into three main subclasses. The first is Phyllocarida, which includes the order Leptostraca. These are considered the most primitive members. They can be found from the intertidal zone to the deep ocean.

The second subclass is Hoplocarida. This contains only the order Stomatopoda, known as mantis shrimps. They are powerful predators with raptorial claws.

The third and largest subclass is Eumalacostraca. This contains the vast majority of the 40,000 species.

Eumalacostraca is further split into three superorders. These are Syncarida, Peracarida, and Eucarida. Syncaridans are mostly small and live in freshwater or underground. Peracaridans are known for having a marsupium to brood their young. This group includes isopods and amphipods.

Eucarida includes many famous animals like lobsters, crabs, and prawns. These animals are found in marine, freshwater, and even terrestrial habitats. Some crabs and woodlice have even moved onto land.

Malacostracans play vital roles in many ecosystems. Most species are scavengers that eat dead organic matter. However, some are carnivores, such as the mantis shrimp. Others, like porcelain crabs, are filter feeders. They are abundant in all marine environments. They are also found in freshwater and on land. This wide reach makes them a central part of many food webs around the world.