Write the characteristics of Phylum Coelenterata (Cnidaria).

(N/A) The characteristics of Phylum $Coelenterata$ (also known as $Cnidaria$) are as follows:
$1$. Habitat: They are mostly aquatic,mostly marine,sessile or free-swimming,and radially symmetrical animals.
$2$. Body Organization: They exhibit tissue-level organization and are diploblastic (composed of two germ layers: ectoderm and endoderm).
$3$. Body Cavity: They possess a central gastro-vascular cavity with a single opening called the $hypostome$ (mouth on the oral surface).
$4$. Specialized Cells: They contain $cnidoblasts$ or $cnidocytes$ (stinging cells) on the tentacles and body,which are used for anchorage,defense,and capturing prey.
$5$. Digestion: Digestion is both extracellular and intracellular.
$6$. Body Forms: They exhibit two basic body forms: the polyp (sessile and cylindrical,e.g.,$Hydra$,$Adamsia$) and the medusa (umbrella-shaped and free-swimming,e.g.,$Aurelia$ or jellyfish).
$7$. Alternation of Generation: Some species exhibit $metagenesis$ (alternation of generation),where polyps produce medusae asexually and medusae produce polyps sexually.