The class Malacostraca is in the Phylum Arthropoda, which is the largest phylum in the Kingdom Animalia. This class includes more than 20,000 species, which is about 2/3 of all the species in the Subphylum Crustacea. Malacostracan is well known as one of the famous exoskeleton organisms. Its name, malacostraca, means ‘soft shell’ in Greek.
It contains the Order Decapoda (crab, lobster, true shrimp, etc) which lay people recognize as crustaceans. It also contains the Order Isopoda, such as woodlice and sowbugs, which is the only land-based crustaceans.
It is very hard to generalize about this class because it is diverse and varied. 
Click picture to see general malacostracan bodyplan.
Malacostracans have a hard exoskeleton typical of crustaceans. Their body is divided into three parts – cephalon, thorax, and abdomen. It may be hard to distinguish between the head and thorax, because they make up the cephalothorax. Every malacostracan except odd species in the Phyllocarida has 5 or 6 segments in its head part, an 8 in thorax, and a 6 in abdomen. The number of odd species is 20, and they have 7 segments in the abdomen. The head has not only mouthparts, but also a pair of antennules and a pair of antennae. They have 10 walking legs and the first pair sometimes forms pincers. Some of them use their abdomen for swimming.
Their typical carapace protects internal gills. Large organisms normally have a well-developed circulatory system. A brain is located near the compound stalked or sessile eyes. It is connected to a few of ganglia through a paired ventral nerve cord. Green glands, in the second antennae, accomplish excretion and osmoregulation. They have a gut for digestion, and a posterior anus on the telson for expelling waste.
Each malacostracan has a specific gender. The female has gonads on its sixth thoracic segment and the male has them on its eighth. The anterior abdominal appendage in a male is used as a reproductive structure which is good at sperm delivery. Some species, such as the peracaridans, brood eggs behind the thorax. Other malacostracans lay eggs. Most species have a nauplius larva stage and the eggs of some species hatch into zoea larvae.
They live in marine water, freshwater, and even on land. Most malacostracans are very active hunters and eat only meat. Organisms which represent many orders have thoracic appendages for catching and crushing food or spearing victim. Several species are parasitic, but they are also scavengers. Herbivorous (organisms which eat plants) also exist.
Most Malacostracans are very active, but there are some benthic burrowing species that remain fairly inactive. 
- Malacostraca Wikipedia
- Class Malacostraca (crabs, krill, pill bugs, shrimp, and relatives) Animal Diversity Web
- Introduction to the Malacostraca University of California Museum of Paleontology
Woodlouse on the bridge.