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AnnelidaLophotrochozoaAnnelids include Marine Worms Class Polychaetes earthworms Class Oligochaeta and leeches Class HirudineaBilaterally symmetric Closed circulatory system which includes five heartsearthwormEmbryological studies indicate that the head composed of the prostomium and peristomium and pygidium are not true segments and in some annelids segmentation has completely disappeared and its absence is a derived characteristicSetal hairs formed from betachitin which extends from the body wallJust like segmentation their absence in leeches is a derived characteristicComplete digestive tract and the straight tubular gut is often modified into regions specialized for different digestive tasksOpenings to the metanephridia collect coelomic fluid from the body cavity and filter it removing nitrogenous wastesurea in terrestrial species and aquaticammonia in wormsGas exchange occurs across the body wall modified into gills in some species Large annelids use respiratory pigments to increase the oxygen carrying capacity of blood that flows in a closed circulatory systemThe nervous system is well developed with a paired ventral nerve chord and an anterior brain that lies on top of the pharynx Sensory structures eyes and tentacles are found on the heads of swimming and predatory annelidsAnnelids include monoecious and dioecious species and development is typical of other spiralian protostome blastopore forming the mouthformation of the body cavity by schizocoely species and is either direct or indirect with a trochophore larval stageMetamerismAnnelids have done something a little different with the coelom in order to become larger animals they repeated the coelomic unit end to end adding the new segments between the mouth and the anusThe proper term for the process is metamerisation each of the segments is a metamere and this is what gives the annelids their unique segmented look
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