The crustaceans are often called the "insects of the sea," as crustaceans dominate the aquatic environment, as insects dominate the terrestrial environment. Arthropods are now believed to be polyphyletic, and the mandibles found in crustaceans were derived separately from the mandibles in the insects. Both the crustaceans and uniramids have antennae, mandibles, and maxillae on the head. Crustaceans have two pairs of antennae, a pair of mandibles and a pair of maxillae on the head. In most crustaceans, there is fusion, forming a cephalothorax. Thoracic and abdominal segments bear appendages for swimming or walking. These appendages are biramous. Crustaceans are primarily marine, but there are many important freshwater species, and even a few terrestrial forms. Respiratory organs, if present, are gills.

The largest group of crustaceans is class Malacostraca. Segments are called caridoid facies, comprised of a head, thorax, and abdomen. The nonsegmented rostrum is at the anterior end, and the tail is composed of the telson and uropods in many species. If the exoskeleton of the head is fused with some or all of the thoracic and abdominal somites, it is called a carapace, which may cover some or all of the rest of the body, as is seen in Daphnia.

Crayfish are often used as a crustacean to study in zoology courses. Much of the text of this summary pertains to the crayfish. The carapace covers the head and thorax. Nearly all somites bear appendages. Each appendage has a basal portion, the protopod, the medial endopod, and the terminal exopod. These are, in the primitive state, biramous. The internal structure of the crustaceans shows metamerism. The major internal body space is the hemocoel, which is not lined with mesoderm. The musculature is arranged in antagonistic groups-flexors and extensors. Respiration in small crustaceans occurs via diffusion across the body surface, but larger forms have gills. Like other arthropods, crustaceans have an open circulatory system. The heart is located dorsally. The hemolymph is colorless, and includes ameboid cells. Hemocyanin or hemoglobin are the respiratory pigments. Excretory organs include antennal glands, maxillary glands, and the epidermal surface. The brain is composed of supraesophageal and subesophageal ganglia with a double ventral nerve cord. Statocysts are found at the base of the first antennae and aid in equilibrium. Compound eyes, if present, are composed of numerous ommatidia, each of which forms a visual image.

Crustaceans typically are dioecious, and even those such as barnacles, that are monoecious, cross-fertilize. The most commonly occurring larva is the nauplius, although some crustaceans have direct development. Periodic ecdysis is necessary for the crustacean to increase in size. Before molting, the epidermal cells swell and separate from the membranous layer, secrete a new epicuticle, and begin secreting a new exocuticle. Enzymes dissolve the old endocuticle, and some of the salts are resorbed. Ecdysis occurs more rapidly when the animal is young, as growth is rapid. Molting is controlled by the molt-inhibiting hormone produced by the X-organ, and the molting hormone, from the Y-organs.

Feeding habits vary between species and life stages of the animal. Many are suspension feeders, others are predators and scavengers. The remipedes are in class Remipedia. They have very primitive characteristics. There are many thoracic and abdominal segments, all with paired biramous appendages. They are small animals, and this class also has a small number of species. Class Cephalocarida is also a very small group, with primitive characteristics. They are hermaphroditic.

Class Branchiopoda contains four orders. The anostracans are the fairy shrimp and brine shrimp and lack a carapace. The notostracans are the tadpole shrimp and have a very large dorsal carapace. The conchostracans are the clam shrimp that are enclosed in a bivalved carapace. The cladocerans are the most abundant of the branchiopods, and are the water fleas that are enclosed in a carapace, except for their head. These crustaceans have flattened legs referred to as phyllopodia which are their respiratory organs. Most are freshwater residents, and often reproduce only by parthenogenesis. Members of class Maxillopoda include a monophyletic group, but which have a variety of different characteristics. When a nauplius larva is present, it has a unique structure called the maxillopodan eye.

The members of subclass Ostracoda are enclosed in a bivalved carapace. They are actually very common-just look in a pool of standing water for tiny animals swimming around that look like seeds. Subclass Mystacocarida includes small crustaceans that live between sand grains in the shallow waters of oceans. In contrast, subclass Copepoda is a very large group, albeit composed of small organisms. The copepods lack a carapace, but retain the single nauplius larva in the adult. Ecologically, they are important primary consumers and carnivores in freshwater and marine habitats. Some are parasitic (called fish "lice") and yet others are intermediate hosts for worms that may parasitize humans. Subclass Tantulocarida is the most recently described subclass of crustaceans. They are ectoparasites of deep-dwelling crustaceans. Members of subclass Branchiura are a small group of fish parasites, but lack gills.

Subclass Cirripedia includes the barnacles that are, as adults, enclosed in a calcareous shell. A stalk attaches gooseneck barnacles, and acorn barnacles are cemented directly to the substrate. All barnacles are marine. They are hermaphroditic and undergo an interesting metamorphosis. They become free-swimming cyprid larvae after hatching. They attach to the substrate via their antennae and metamorphose. The plates are formed, and their swimming appendages change into the feeding cirri. Some barnacles are parasitic. Class Malacostraca is the largest class of crustaceans with respect to number of species, and also includes the largest members in size. Members of order Isopoda are common in marine and freshwater habitats, and are among the few crustaceans to become truly terrestrial (pill bugs). They are dorsoventrally flattened, lack a carapace, and nearly all appendages are similar to each other. The crustaceans in order Amphipoda are similar to the isopods, but the amphipods are laterally compressed, and their limbs differ in form and function. Order Euphausiacea is a group of relatively small crustaceans, and the group is not particularly speciose, but may be very abundant in the ocean, known colloquially as krill. Most are bioluminescent and form huge swarms, particularly in polar regions. They form the food base for the short food chain that ends with filter-feeding fish and whales. Order Decapoda is so named as all have five pairs of walking legs. This group includes crayfish, crabs, shrimp, and lobsters. These crustaceans are pivotal in the ecology of many aquatic communities, and are economically important to humans.

The phylogeny of this group is not yet certain. Initially, as they have mandibles, crustaceans were lumped with the insects and allies. But the mandibles appear to have different structures, so the development of mandibles may be a convergent character. The remipedians seem to exhibit the most primitive characters.