Tissue Layers Recall that a blastula has a single layer of cells surrounding a fluid-filled cavity. In the embryo, this layer is often referred to as the blastoderm and the internal space is the blastocoel. Eventually, the blastoderm develops into the ectoderm which in the adult stage becomes the epidermis. Sea sponges are the only animals that have only a single tissue layer and are thus monoblastic. They do not undergo gastrulation and lack a gut.Some organisms only develop the two embryonic tissues shown in Step B, and therefore have two adult tissue layers. These are diploblastic animals. An example of a diploblastic animal is a cnidarian or sea jelly.A third tissue layer (mesoderm; Greek: mesos meaning "middle" and deros meaning "skin") develops in most animals, making them triploblastic. Mesoderm forms from the endoderm in one of two ways, neither of which will be covered here. In the adult animal, the mesoderm eventually develops into muscles, an internal skeletal system, and a circulatory system. Question 9 Cnidarians lack an internal skeletal system. Using the information on tissue layers above, explain why this is so. Your answer