A body plan can be thought of as a cross-section through an animal, showing only the most fundamental arrangement of the tissue layers. It does not show any detail, such as the position of the internal organs.
Here is a picture showing the increasing complexity of the three main body plans:
- Diploblastic acoelomate
- Triploblastic acoelomate
- Triploblastic coelomate
All phyla fit into one of the three body plans shown in the diagram above.
When we talk about complexity in phyla, we refer to the number of tissue layers and whether it has a coelom (see below).
The more tissue layers, and the presence of a coelom the more complex the animal. Humans, for example, have a coelom and are triploblastic (see below), making them one of the most complex organisms in terms of their body plan.
Diploblastic: An animal possessing 2 major tissue layers. These include the outer layer (the ectoderm) and the inner layer (the endoderm).
Triploblastic: An animal possessing 3 major tissue layers. It has a middle layer (the mesoderm), between the endoderm and the ectoderm.
Radial Symmetry: Animals having symmetry around a central axis. Animals with radial symmetry are diploblastic.
Bilateral Symmetry: Symmetry in which the body can be divided into 2 mirror-image halves.
Coelom: Fluid-filled cavity within the mesoderm. It is not the gut. Having a coelom gives the animal certain advantages:
- It enables independent movement of the gut wall and the body wall.
- It provides space of the enlargement and development of internal organs.
- It may act as a circulatory medium for transport of materials or a storage area of excess or waste materials.