1. They show metamerism. It is modified by the specialization of body regions for specific functions (tagmatization).
2. Chitinous exoskeleton provides support and protection. It is modified to form sensory structures.
3. They have paired jointed appendage.
Phylum arthropoda have four sub-phylum:
A. Subphylum Trilobitomorpha
They are Marine, They are all extinct now. They lived from Cambrian to carboniferous periods. Their bodies are divided into three longitudinal lobes: head, thorax and abdomen. They have one pair of antennae. They have biramous appendages (each appendage has two branches).
B. Subphylum Chelicerata
Their body is divided into prosoma and opisthosoma. Their first pair of appendages is chelicerae, It is piercing or pincer like . It is used for feeding.
1. Class Merostomata
They are marine. They have book gills on opisthosoma. This phylum has two subclasses:
Example: Water scorpions, the horseshoe crabs
2. Class Arachnida
They are mostly terrestrial. They have book lungs, tracheae or both. They have four pairs of walking legs in adults.
Examples: Spiders, scorpions, ticks, mites.
3. Class Pycnogonida
They have reduced abdomen. They have no special respiratory or excretory structures. They have four to six pairs of walking legs. They are common in all oceans.
Example: Sea spiders
C. Subphylum Crustacea
They are mostly aquatic. Their head have two pairs of antennae, one pair of mandibles and two pairs of maxillae. They have biramous appendages.
1. Class Remipedia
It has a single species. Their body with approximately 30 segments. These segments bear biramous appendages.
They are small marine crustaceans. They have uniform, leaf like, triramous appendages.
3. Class Branchiopoda
They have flattened, leaf like appendages. These appendages are use in respiration, filter feeding, and locomotion. They are found mostly in freshwater.
Example: Fairy shrimp, brine shrimp. clam shrimp, water fleas.
4. Class Malacostraca
Their appendages are modified for crawling, feeding, swimming. Examples: Lobsters, crayfish, crabs, shrimp. isopods (terrestrial).
5. Class Copepoda
Their maxililpeds are modified for feeding. Their antennae are modified for swimming. Example: Copepods.
6. Class Cirripedia
They are sessile as adults. They are marine. Their body is enclosed by calcium carbonate valves, Example: Barnacles
D) Subphylum Uniramia
Their head have one pair of antennae. They have one pair of mandibles. Their all appendages are uniramous. It includes insects and their relatives.