The arthropoda are also known as the animals with jointed feet. Aristotle described it and Lamarck divided it into three classes such as crustacean, hexapoda and arachnida. Von Seibold proposed the arthropoda. They are found everywhere on land, soil, sea and fresh water and also acts as a parasite. Their body is divided externally and has jointed legs with a distinct head. They arise from the three germ layers such as ectoderm, mesoderm and endoderm. They are known as triploblastic animals. They have unisexual and bisexual forms and show sexual dimorphism. They have bilateral symmetry. They have an organ system level of organization. Their body wall is made up of cuticle which forms the exoskeleton. The digestive system is complete and has mouth and anus for ingestion and egestion. It includes fore gut, mid gut and hind gut. The blood vascular system is of open type with the presence of irregular spaces known as lacunae. They have reduced coelom and have cavities in reproductive and excretory organs which are filled with the blood and are known as haemocoel. The gills act as a respiratory organ in the aquatic forms and book lungs act as a respiratory organ in the terrestrial forms. The cutaneous form of respiration also occurs. They have the green glands for excretion and there are no nephridia. The green glands are also known as malphigian tubules. The nervous system is ring like with the presence of nerve cord and ganglia. They have no cilia and have compound eyes which develop mosaic vision. They also have statocysts. Their muscles are stripped. They have endocrine glands for secreting hormones. Some of them secrete pheromones and also show metamorphosis. They show parental care and also show parthenogenesis in which egg develops to an individual without fertilization by sperm. This phylum has the largest number of animals. Their fossils are known as trilobites.
The arthropoda are better than annelida as they have compound eyes, distinct head, stripped muscles, endocrine glands, joint appendages, respiratory organ and pheromones.
