Arthropoda

Arthropods are  cosmopolitan .They are found almost in every habitat. They are found in desert ,inside soil, in freshwater, in marine water, in snowy areas and also as parasites. Most of these are solitary or gregarious. some form colonies with social organisation.Body is divided into 3 regions head ,thorax and abdomen. In some species head is fused with thorax  to form cephalothorax .Segmentation is external. lnternal septa are absent. Symmetry is bilateral. Coelenterates are triploblastic animals with organ level of body organisation . Spaces in between organs merge and form blood filled cavity called haemocoel.Head is formed by fusion of 6 segments .it has various types of mouth parts. Eyes are compound . Exoskeleton is mostly made up of chitin. it undergoes moulting or ecdysis at intervals . Insects have wings made up of chitin. they have jointed legs and other appendages. Digestive canal is complete . Blood circulatory system is open type. Blood vessels cannot be differentiated into arteries and veins. Blood is colourless and is called hemolymph . Excretory system consists of green glands, malpighian tubules. gills, tracheae,book gills, book lungs, respiratory siphon are respiratory organs found in various arthropods. Nervous system consists of various  ganglia ,nerves and nerve plexus. Most arthropods are unisexual, few are hermaphrodites. fertilization can be external or internal. Parthenogenetic development of eggs into young ones is found in many arthropods like Honey Bee cockroach grasshopper etc. Asexual reproduction is absent. Development may be direct or indirect involving one or more larval stages. series of changes that convert larvae into adults are together called as metamorphosis.