(After Snodgrass, 1938, 1960)
Arthropods are bilaterally symmetrical, metamerically segmented animals having chitinous exoskeleton. Moulting is necessary for growth. They possess jointed appendages, a haemocoel or schizocoelom and have open circulatory system. Segments as well as their appendages are specialized to form various organs. Arthropods constitute about 80% of all animals.
Fossil arthropods having body divided into 3 longitudinal lobes, one median lobe and two lateral pleural lobes.
Appendages were biramous and had gills attached on them. They were abundant during Cambrian and Ordovician periods. Body was divided into cephalon, trunk and pygidium. They also possessed segmented antennae and a pair of compound eyes.
First appendages are modified as chelicerae for feeding and second ones modified as pedipalps. Body is divisible into prosoma and opisthosoma, the latter is sometimes divided into mesosoma and metasoma. There are no antennae in these arthropods.
Body is divisible into prosoma, mesosoma and metasoma. They are marine scavengers in which abdominal appendages carry gills and telson is long spike-like.
This group includes extinct giant scorpions of Ordovician to Permian periods. They had 5 pairs of thoracic swimming appendages and 7 pairs of abdominal appendages which carried 6 pairs of gills. Metasoma was without appendages. Ex. Pterygotus; Eurypterus. They were the dominant predators of the Palaeozoic era.
There are only 5 surviving species of horse-shoe crabs in the world and hence they are called living fossils. They have 5 pairs of thoracic walking legs which are chelate and armed with spines. There are 6 pairs of abdominal appendages carrying 5 pairs of book gills for respiration. Ex. Limulus or king crab.
There are one pair of chelicerae and one pair of pedipalps and 4 pairs of thoracic legs in these arthropods. Abdomen is without legs. Breathing takes place through the book lungs or tracheal system or both. Ex. Scorpions; pseudoscorpions; spiders; ticks and mites.
Class PYCNOGONIDA (=PANTOPODA)
They are commonly known as sea spiders that measure 1-10 mm in length. Cephalothorax is large and segmented while abdomen is highly reduced. Legs long for crawling on the sea bottom. They are carnivores that feed on cnidarians and worms. Ex. Nymphon; Pycnogonus.
Arthropods with strong mandibles for chewing and cutting, or mouth parts are modified for different modes of feeding. They are terrestrial as well as aquatic animals.
They possess two pairs of antennae and their body is divisible into cephalothorax and abdomen. Eyes are simple or compound. Appendages are variously modified.
Horse-shoe shaped body without a carapace and eyes. They are marine bottom dwellers having long antennae for food gathering. Ex. Hutchinsoniella.
They are small freshwater forms with shield-like carapace and abdomen without appendages. Thoracic appendages are used for respiration. Ex. Apus; Daphnia.
Marine and freshwater animals with laterally compressed body enclosed in a bivalve shell. Two pairs of thoracic appendages are present. Feeding and locomotion is performed by head appendages. Appendages also have respiratory function. Ex. Cypris; Cypridina.
Body is divided into head, thorax and abdomen. Head appendages are bushy. Four pairs of thoracic appendages are present. They are filter feeders, found among sand grain in beaches. Ex. Deirocheilocaris.
Body is composed of cephalothorax and abdomen, the latter has no appendages. Single eye is present and there is no carapace. Antennae beat in rotary fashion for swimming. Some are planktonic but a large number are parasitic. Ex. Cyclops; Caligus, Monstrilla.
These small arthropods possess dorsoventrally flattened body adapted for ectoparasitic mode of life on fishes. Thoracic appendages are 4 pairs, used for swimming. Maxillae are used for anchoring on host. Maxillipedes are absent. There is a spine for puncturing the skin of fish. Gills are absent and abdomen has respiratory function. Ex. Argulus.
They are marine, either sedentary or parasitic. Compound eyes are absent. Thoracic appendages are 6 pairs and abdomen is rudimentary. They are commonly known as barnacles. Ex. Lepas; Balanus; Sacculina.
Large crustaceans that have 5-segmented head, 8-segmented thorax and 6-segmented abdomen. Cephalothorax is covered with carapace. Compound eyes are stalked. This group includes 75% of all crustaceans. Ex. Mysis; Squilla; Palaemon; Cancer.
They possess long, segmented and worm-like body divided into cephalon and trunk. Trunk appendages are biramous. Two pairs of maxillae and one pair of maxillipedes are present for feeding. Found in marine caves in Bahamas. Ex. Speleonectes; Lasionectes.
They are parasitic in the respiratory system of reptiles in USA, Europe and Australia. Body is worm-like with 5 appendages on the anterior side and 2 pairs of hooks for anchoring. Trunk is ringed without appendages. Ex. Linguatula; Cephalobena.
They are ectoparasites on deep water crustaceans. There are no appendages in adult which has a bag-like body. Larva is free swimming and bears 6 or 7 abdominal somites and appendages. Adult penetrates the host skin by a mouth tube. They look similar to copepods. Ex. Basipodella; Deoterthron.