Biologically speaking, cnidarians are evolutionarily successful organisms. Not only are they numerous as individuals, but have many species. Moreover, cnidarians live all over the Earth's water supply. Their evolutionary aspects have no doubt led to their success.
Cnidarians are simple organisms. An example of a cnidarian is a hydra. They are multicellular, but their development can be classified as highly as the tissue level; they do not possess any organ structures. Their incomplete digestive system is composed of a cavity with a single opening for the entry and exit of food and waste, respectively. They exhibit a radial symmetry and lack any body cavity. Simplicity seems to be the theme for cnidarians. They perform gas exchanges by diffusion. They sense their environments by a rudimentary nervous system composed of a nerve net. Their simplicity is their key to success. Perhaps the straightforwardness of their body design leads them to concentrate on their sole goal of acquiring food and reproducing. Further, their simple body gives them a great capacity for regenerating if injured. They are, however, limited by their immobility. Yet without their ability to move, it is their mere simplicity that gives them their abilities that promote their survival.
Arthropods are also very successful organisms. They outnumber cnidarians in terms of individuals, species, and diversity. They are more complex organisms that permit more diversity among them. A lobster or a crayfish is an example of an arthropod.
Arthropods are more advanced them cnidarians. They have moved up the evolutionary ladder and possess organs. Further, they have a coelom, a body cavity, and an exoskeleton to protect their organs. They possess a complete digestive system as well as a closed circulatory system. Their nervous system is made of a brain and a nerve cord. Furthermore, they possess appendages designed for movement.