Subkingdom Eumetazoa (all the other phyla)
The Eumetazoa consist of all the other animal phyla.
The Eumetazoa have been divided into two major branches based on body symmetry.
Branch Radiata
The phyla ion this branch have radial symmetry. These animals have an oral (top) and aboral (bottom) side, but no front, back, left or right sides. This branch includes the phylum Cnidaria, (the jellyfish, sea anemonies, and corals).
Branch Bilateria
The phyla in this branch have bilateral symmetry. Bilaterally symmetrical animals have dorsal (top), ventral (bottom), anterior (head), posterior (tail), left and right body surfaces. These animals exhibit cephalization (an evolutionary trend toward concentration of sensory structures at the anterior end). This branch includes all other animal phyla.
Care must be taken when assigning an animal to an evolutionary line. Some such as the Phylum Echinodermata, show secondary radial symmetry which evolved as an adaptation to their lifestyle.
Diploblastic vs. Triploblastic
There is another characteristic of the body plans of the radiata-bilateria lines that distinguishes them better than symmetry. This is the pattern of development of the embryos, and the production of embryonic tissues called "Germ layers".
Germ Layers
The early embryo of all eumetazoans undergoes gastrulation. Concentric germ layers develop which form the various tissues and organs as development continues. All Bilaterally symmetrical animals develop three germ layers (ectoderm, mesoderm, and endoderm) and are termed triploblastic. The radiata develop only two germ layers, (ectoderm and endoderm but no mesoderm) and are therefore termed diploblastic (e.g., phylum Cnidaria)
Three Germ Layers of an Early Embryo
Ectoderm: This layer covers the surface of the embryo. It forms the outer covering of the adult animal and the central nervous system in some phyla.
Endoderm: This is the innermost germ layer which lines the archenteron (primitive gut). It forms the lining of the digestive tract and outpocketings give rise to the liver and lungs of vertebrates.
Mesoderm: This layer is located between the ectoderm and endoderm in triploblastic animals. It forms the muscles and most organs located between the digestive tract and outer covering of the animal.
With these three layers of tissue and the types of organs they develop into in the adult animal we can now see how the two major animal lines differ in their basic body plans.
Above you see a "Blastula" and below the process of "gastrulation".
In this diagram above you can see the development of the diploblastic embryo.
PHYLUM - CNIDARIA: Diploblastic radiata
This phylum contains over 9,000 extant animals. Most of them are marine and all of them are extremely primitive. As a marine group, they are reasonably successful animals. They are often large colourful organisms that are either free swimming or sessile. The attached animals occur either singly or in enormous marine colonies. They represent one of only two groups of animals that are primarily radially symmetrical. This feature is normally associated with a sedentary life style. The phylum is diploblastic but has not advanced beyond the tissue level of organisation.
The phylum Cnidaria takes its name from the presence of unique stinging cells called cnidocytes. The phylum includes such animals as plant like hydroids, colourful sea anemones, graceful jellyfish and the reef forming corals.
A. Phylum Characteristics:
1. Multicellular, metazoan animals which are radially symmetrical about an oral - aboral axis.
2. Two types of body forms, medusae and polyps.
3. The animals have reached the tissue level of organisation.
4. The bodies are derived from two embryonic cell layers (diploblastic), the ectoderm and endoderm which are separated by a third gelatinous layer called the mesoglea (this is not the same as "Mesoderm").
5. The main body cavity, the gastrovascular cavity has a single opening that serves as both the mouth and anus.
6. Support of the organisms is generated either through a calcareous exoskeleton or through a rudimentary hydrostatic skeleton.
7. The inner, outer or both surfaces are possess special stinging cells, cnidocytes.
8. Locomotion and co-ordination are generated by epithelio-muscular cells and a primitive nerve net.
9. The animals have no excretory, circulatory or respiratory systems.
10. Reproduction is by asexual budding or sexual reproduction.
here you can see a hydra (polyp form) with two asexual buds, one slightly older than the other.
Here is a photo of a jellyfish (medusa form).
