Diploblasty is the condition of blastula in which there are 2 primary germ layers: the ectoderm and the endoderm.
Cnidaria are diploblastic animals; in other words, they also have 2 main cell layers, while more complex animals are triploblastic having 3 main layers.
The two main cell layers of the cnidarians form epithelia which are mostly one cell thick, & are attached to a fibrous basement membrane, which they secrete.