platyhelminthes--- dorso ventrally flattened body that is why calles flat worms, bilaterally symmetrical, triploblastic, acoelomate, organ level of organisation, absorb nutrients from the body of hosts through their body surface, have specialised cells called flame cells for osmoregulation and excretion, sexes not seperate, fertilization is internal and development is indirect i.e. through larval stages. eg- taenia (tapeworm)
annelids-- organ system level of organisation, bilaterally symmetrical, triploblastic, metamerically segmanted, coelomate, posses longitudnal and circular muscles which help in locomotion ( nereis which is aquatic posses parapodia for swimming i.e. locomotion), closed circulatory system, nephridia for osmoregulation and excretion, nereis is dioecious (sexes seperate) but in leech and earthworm sexes are not seperate i.e. monoecious, reproduction is sexual.
arthropoda-- organ system level of organisation, bilateraly symmetrical, triploblastic, segmented, coelomate, body is covered with chitinuous exoskeleton, body consist of head thorax and abdomen, jointed appendages, respiratory organs are gills, gill slits, book gills, book lungs or tracheal system, open circulatory system, sensory organs like antennae, eyes, statocycst or balance organs are present, excretion through malphigian tubes, dioecious, fertilization internal, oviparous, development direct or indirect. eg- apis(honey bee)
for rest you may prefer your NCERT book or they are also given in meritnation study material.
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