Crustaceans are sometimes called the insects of the sea, because they are both Mandibulate Arthropods and because, while the insects may rule on land, the crustaceans rule in the water. The other great group of Arthropods are the Chelicerates, which are animals such as spiders and scorpions, mites and ticks. Both chelicerae and mandibles are biting mouthparts but they develop from different segment appendages in the two groups. Mandibulates, both insect and crustacean, are characterised by the following head appendages: a. a pair of preoral antennae which are not found on chelicerates at all, and b. the mandibles are formed on the second postantennal segment followed by maxillae on the next two segments. In the chelicerates, there are no antennae, instead the claw-like chelicerae develop on the second segment and are the first appendages on these animals.
As well as mandibles, Crustaceans have the usual hard arthropod exoskeleton which has to be moulted in order for the animal to grow and, of course, jointed legs. After this Crustaceans veer away from insects and have enough important differences related to both evolutionary choices and lifestyles to be placed in separate groups. The major crustacean life style adaptation is to have gills for extracting oxygen from water. They have five pairs of head appendages: first antennae, second antennae, mandibles, first maxillae and second maxillae. As well, crustaceans have a variety of thoracic appendages adapted for many purposes from swimming to walking to clasping the other sex, holding eggs, grasping prey or aeration. There are reduced abdominal appendages but those that exist are different than those on the thorax.
Crustaceans have three main body parts, head, thorax and abdomen, but the segments in each do not always correspond with those of insects and in many crustaceans the head and thorax are covered by a carapace. Segments are called somites in crustacea and there are a lot of other terms specifically for crustacean anatomy, which makes the subject even more confusing. There is a posterior somite called the telson instead of the tail The telson contains the anus (that name at least hasn't changed) and has no appendages, although the telson itself is often flattened into appendage-like lobes.
Internally, Crustaceans have a typical arthropod digestive system, nervous system, heart and circulatory system. There is a median eye plus usually a pair of lateral eyes, often on stalks. Sexes are usually separate, there is indirect development of the embryo and most crustaceans go through a ciliated, planktonic larval stage called a nauplius.
Crustacean appendages are so many and varied because they are the crustaceans' tools for whatever lifestyle they are following. As such, they show extensive adaptive radiation and are not always recognisable as being from the same source in different species. For this reason alone, they are important in crustacean taxonomy and classification. Unfortunately there is an specialised vocabulary associated with all the different parts and a good Invertebrate Zoology text should be consulted if you are trying to identify a crustacean by its body parts.
Class Crustacea is divided into a number of subclasses. The first seven subclasses used to be grouped together as the Entomostraca. Subclass Cephalocarida are inconspicuous animals in inconspicuous places. They are tiny and eyeless and have primitive characteristics that indicate they are descendents of some of the first crustaceans. The subclass Mystacocarida is similar, small and primitive and showing characteristics that appear to be intermediate between two more major groups, the copepods and the branchiopods. The much larger subclass Branchiopoda are among the most primitive modern crustaceans whose fossils first appear in the Paleozoic.
The fourth subclass of the Entomostraca is the Branchiura or fish lice. These are highly modified Crustaceans which live on the gills or body surfaces of fish and amphibians. The fifth subclass is the Ostracoda, crustaceans that have two shells and superficially resemble molluscs. Another subclass whose members superficially resemble molluscs is the Cirripedia, the barnacles. The most successful subclass of this group is the Copepoda. These are the most typical looking of the crustaceans so far and are abundant in many habitats as well as having some successful parasitic forms.
The rest of the Crustacea and indeed two thirds of all species belong to the other group, the Malacostraca. If you think of a typical lobster or prawn or crayfish, this is a Malacostracan. Phyllocarida are the most primitive of the Malacostraca and are known from fossils as far back as the Cambrian, but there is only a single modern order. The rest of the Malacostraca are grouped into superorders: 1. Syncarida, a group of primitive forms, many of which are in restricted or unusual habitats like caves. 2. Hoplocarida, the mantis shrimps, which resemble aquatic praying mantids with large raptorial legs. 3. Peracarida, a large superorder which includes mysid shrimp, isopods (pill bugs) and amphipods plus some smaller orders.
The fourth and most important suborder is the Eucarida, which includes the shrimp-like euphausids and the Decapods. Order Decapoda contains two suborders: the Natantia or swimming decapods and the Reptantia or walking decapods. Natants include the penaeid prawns and shrimp, one of the most economically important groups in the crustacea, plus the carids, which are all the fresh-water shrimp plus many deep sea forms. Most natantia are predators or scavengers.
Order Reptantia contains the lobsters, crayfish and crabs. They are divided into three suborders. The Macrura are the economically valuable lobsters and crayfish plus the marine burrowing shrimps and ghost shrimps. The Anomura are the most primitive forms of crabs, including the Galathea crabs, porcelian crabs and hermit crabs. In the Anomurid crabs, the abdomen is shorter than in the crayfish types but not as small as the true crabs, which belong to the third suborder, Brachyura. The brachyuran crabs are the peak of crustacacean evolution according to Meglitsch (p. 574) in both structure and behaviour. All the commercially important crabs are brachyurans as are all the common little crabs seen in intertidal zones and mangrove swamps around the oceans of the world.
As can be seen in this short introduction, the crustaceans are a bewilderingly diverse group of organisms and important ones too, both as members of their ecosystems and as important food sources for humans.
Reference: Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press.