Throughout these invertebrate pages you will find reference to animals as being Coelomate, Pseudocoelomate and Acoelomate. All these terms, and others explained below are to do with the nature of the body cavity of the animal. Simple animals do not have a body cavity but higher animals like mammals do. In mammals the body cavity is called a Coelom and it is the area that contains all heart and lungs as well as the gut which is a cavity in its own rite. The words used to define different body cavities relate to how the cavity comes into being during the development of the embryo as well as to its final observable structure. Below university level education it is not terribly important to understand the differences but I have put these explanations up separately from the glossary to make it easier to use. Also because of the length of this explanation I am only typing it out once here rather than each time I have used such a term.
Diploblastic animals have two cell layers to their bodies, an outer Ectoderm and an inner Endoderm, between these two layers may be an amount of noncellular material. Triplobastic animals have 3 cell layers in their bodies, Ectoderm (Outer layer) Mesoderm (middle layer) and Endoderm (inner layer), simpler animals have only 2 cell layers in their bodies and are called diploblastic. Body cavities of any sort only exist in triploblastic animals. These layers form in the embryo during a process called gastrulation and later give rise to different parts of the body. Thus the Ectoderm will eventually become, the epidermis of the skin, sweat and sebaceous glands, epidermal coverings (hair, feathers, scales, horns etc.) and the nervous system including those parts of the sense organs that are sensory, rather than supportive. The mesoderm gives rise to the skeleton, the muscles, the dermis of the skin, blood and blood vessels, mesenteries and the lining of the coelomic cavity. From the endoderm arise the lining of the guts, lungs and urethra as well as the urinary bladder, the thyroid, parathyroid and thymus and the secretory parts of the liver and pancreas.