Because they are grouped in the same kingdom, the nine animal phyla share the same fundamental characteristics- they are multicellular, heterotrophic eukaryotes that obtain nutrients through ingestion, they lack cell walls, they have nervous tissue and muscle tissue, and they reproduce sexually and have a unique embryonic life cycle. However, the animal phyla have a great number of differences as well. Some are visible to the naked eye, while others are less obvious, and still more cannot even be seen after embryonic development. What are these differences, and how did they shape the development of the phylogenetic tree?
In animals, the embryo becomes layered through the process of gastrulation, or the formation of the two layered, cup-shaped embryonic stage from a blastula. These layers are called germ layers. Most animals are triploblastic, meaning they have three germs layers. These are the ectoderm, the endoderm, and the mesoderm. The ectoderm is the outermost layers which forms the body's covering and the central nervous system. The endoderm is the innermost germ layer. It forms the digestive tube and gives rise to most organs. The third layer, the mesoderm, exists between the endoderm and ectoderm. It forms the muscles and most other organs towards the upper layer of the animal. Diploblastic animals, or animals with only two germ layers, lack mesoderm. There are only two diploblastic phyla: Porifera, the sponges, and Cnidaria, the "bag animals." All other animals are triploblastic. In fact, sponges are such simple animals that they lack even true tissue. They are in the parazoan category (parazoans lack true tissue). Sponges are the only animals that are parazoans- all other phyla are eumetaozoans, animals with true tissue.
The presence or absence of a body cavity is important in distinguishing different phyla. A body cavity is a fluid lined space separating the digestive tract from the outer body wall. It can act as a shock absorber,...
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