Arthropoda is the largest phylum of animal kingdom. Body of arthropod is divisible into head,thorax and abdomen. Head and thorax often fused to form cephalothorax. The respiration takes place by general body surface,gills,tracheae or book lungs. Mollusks are dioecious or monoecious,one or more gonads with gonoducts,opening into renal ducts or to exterior. The fertilization is external or internal,development direct or through free larval forms. Echinoderms have a pentamerous radial symmetry derived from an original bilateral symmetry. The fertilization is external,development indirect through free-swimming larval forms. Chordates are sharphy distinguished from non-chordates by presence of notochord,dorsal tubular central nervous system and pharyngeal gills slits.