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 the 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 non-cellular 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.
Acoelomate animals are called acoelomates and they have no true body cavity.
Pseudocoelomate animals have a pseudocoelom.
They have a body cavity, but it is not lined with mesodermal cells. It exists between the mesoderm and the endoderm that makes up the walls of the gut. The pseudocoelom does not have supportive mesodermal mesenteries.
Coelomate animals have a coelom, this is a body cavity that has a mesodermal lining.
Coeloms arise in two different ways.
Through splitting of the mesoderm cell mass which arises from the walls of the archenteron, in which case it is called a schizocoelom. Or from the invagination of parts of the endodermal aspect of the archenteron, when it is called an enterocoelom.
The coelomate phyla are Entoprocta, Ectoprocta, Phoronida, Brachiopoda, Mollusca, Priapulida, Sipuncula, Echiura, Annelida, Tardigrada, Pentastoma, Onychophora, Arthropoda, Pogonophora, Echinodermata, Chaetognatha, Hemichordata, Chordata.