what do mean by no body cavity between epidermis&gastrodermis?
Body cavity is the space between gastrodermis (endodermal in origin) and epidermis(ectodermal in origin). The space is lined by cells mesodermal in origin.
In other words we, can say that the coelom or the body cavity is the space enclosed by the mesoderm where the internal organs are suspended. The animals that have a true body cavity are known as coelomic or eucoelomic. For example, organisms belonging to phylum Annelida.
In acoelomate animals, the space between the gastrodermis(outer lining of the gut) and the epidermis is filled with unspecialised cells.i.e no body cavity is present. This condition is found in Phylum Cnidaria and Phylum Platyhelminthes.
You can understand with the given diagram of Hydra.