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Phylum : Arthropoda :-
These are the most successful group of animals. They outnumber all other animals in population strength. The body is segmented. It is covered by a hard exoskeleton made of chitin. During growth the exoskeleton is shed (moulting of ecdysis). The legs or paired appendages are jointed. The head region has a pair of prominent compound eyes. Each compound eye is made up of several photoreceptor sub units called Ommatidia.
They have an open circulatory system without vessels. The body cavity is filled with a fluid called haemolymph. Such body cavity is known as haemocoel. These are unisexual, exhibiting sexual dimorphism. The young forms produced are invariably called the larvae. The larvae undergo metamorphosis and develop into adults.
This Phylum comprises five Classes, Class Onychophora: It includes small worm like Peripatus. Peripatus shows Annelidan and Arthropoda characters. Hence this may be considered as a connecting link between the two groups.
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