Definition and Introduction
Bilateral and protostomial eucoelomate eumetazoa with metamerically segmented and each segment bearing a pair of jointed locomotory appendages. Arthropoda literally means animals bearing jointed feet (Gr. Arthron= jointed +podos=foot.) These are extremely successful animals due to a protective exoskeleton and jointed appendages .That is why more than 8.1/2 lac arthropod species are known as against about 2.1/2 lac species of the remaining animals .
Aristotle described a few carbs and other arthropods. Linnaeus included all such animals in his group insect Lamarck divided this group into three class curstacea, Hexapoda and arachnida finally , von seibold (1845) established the phylum arthropoda for these animals.
1. Occur widely on land, in air and in all sorts of water, from snowy tops of high mountains to the depths of ocean. Many are parasite of other animals and plants . Hence the phylum is of great economic importance.
2. Bilateral, triploblastic body segmentd and also regionated into head, thorax and abdomen. Segmentation marked only externally numbers of segments or somites fixed and each has its separate exoskeleton of thick and hard chitinous cuticle secreted by epidermis of body wall. Head somites always fused.
3. Each segment basically bears a pair of lateral jointed appendages adapted for food ingestion , locomotion , respiration , copulation,etc.
4. Muscular system well developed , muscle fibres always striated.
5. Digestive tract complete ,Most head appendages form mouth parts with lateral jaws for chewing or sucking . Anus terminal.
6. Coelon reduced to small cavities n in excretory and reproductive organs replaced elsewhere by blood sinuses which merge together to form a large perivisceral cavity the haemocoel around viscera, Sinuses form an open blood vascular system filled with haemolymph which may contain haemocyanin. Haemocoel communicates with a long tubular and pulsatile mid dorsal heart.
7. Respiration by gills (aquatic forms ) or tracheae or book lungs (terrestrial forms ) by diffusion through body surface in some.
8. Excretion by coelomoducts or specialized green or coxal glands or ny malpighian tubules .
9. Nervous system basically similar to the typical annelid plan head with brain ring which is connected to a double ventral cord. Having paired segmental ganglia which represent true metamerism .Well developed sensory organs or various types.
10. Sexes mostly separate with sexual dimorphism .
11. Fertilization typically internal in female body Eggs megalecithal. Oviparous or viviparous,
12. Life cycle includes one or more larval stages that metamorphose into adults.