Malacostraca

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.