(D) : Cnidarians are sac-like animals that are aquatic. They are mostly marine,but some species like $Hydra$ are found in fresh water. They are the si

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(D) : Cnidarians are sac-like animals that are aquatic. They are mostly marine,but some species like $Hydra$ are found in fresh water. They are the simplest organisms that have attained a tissue level of organization. Members of $Ctenophora$,$Cephalochordata$,and $Echinodermata$ are exclusively marine.