Because after embryonic development. What are these differences,
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.
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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 bodys 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, protecting the organs, regulate circulation, and allow room for organ growth. Most animals that have a body cavity also have a complete digestive tract.
Acoelomates lack a body cavity altogether. An example of an acoelomate is a flatworm. Pseuodocoelomates have a body cavity, but it is not completely lined with mesoderm. The roundworm is an example of a pseudocoelomate. The eucoelomates have a true body cavity. It is completely lined with mesoderm.
A segmented worm, such as an earthworm, is an example of a eucoelomate. The eucoelomates are divided into two categories: protosomes and deuterosomes. A protosome is a member of one of two distinct evolutionary lines of coelomates, consisting of the annelids, mollusks, and arthropods, and characterized by spiral, determinate cleavage, schizocoelous formation of the coelom, and development of the mouth from the blastopore. A deuterosome is one of two distinct evolutionary lines of coelomates, consisting of the echinoderms and chordates and characterized by radial, indeterminate cleavage, enterocoelous formation of the coelom, and development of the anus from the blastopore. In these definitions, it is easy to spot the differences between these two evolutionary lines, but what do they mean? We will start with cleavage. Cleavage is the separating divisions that transform the zygote into a ball of cells. Protosomes undergo spiral, determinate cleavage.
Spiral cleavage means that the planes of cell division are diagonal to the vertical axis of the embryo. (See the bottom of this paragraph for a drawing). Determinate cleavage means that the developmental fate of each embryonic cell is rigidly set. If you were to take one cell away from the blastula, that cell would not develop specified parts because it is not programmed to. Deuterosomes undergo the opposite in terms of cleavage- radial and indeterminate.
In radial cleavage, the planes of cleavage are either perpendicular or parallel to the vertical axis of the egg. (See the bottom of this paragraph for a drawing). Indeterminate cleavage means that each cell produced by early cleavage divisions retains the ability to develop into a complete embryo.
If you were to separate one cell of this type from a blastula, it has the ability to divide itself and from a normal larva. The way in which the mesoderm forms is another way that protosomes and deuterosomes are differentiated. We know that both groups have mesoderm, because they are both coelomates. However, protosomes undergo schizocoelous development, which means that solid masses of mesoderm split to form the coelom, (see the bottom of this paragraph for a drawing), while the coelom forms from the mesodermal outpocketings of the archenteron. The archenteron is the developing digestive tube.
The final difference in protosome and deuterosome development is the blastopore forms. The blastopore is the single opening of the archenteron. In protosomes, the blastopore forms the mouth, while in deuterosomes, the blastopore forms the anus.
Another feature that distinguishes different animal phyla is symmetry. There are two main types of symmetry: radial and bilateral. Radial animals can be divided into top and bottom sides, or oral and aboral, respectively.
They have no head or tail, and no left and right sides. A good example of this type of symmetry is a jellyfish. It is symmetrical when viewed from above or below. Bilateral symmetry is two-sided symmetry. Bilateral animals are symmetrical when cut in half from above or below.
Cephalization in this group has created a clearly visible head, and when divided, the head section is called the animals anterior, the tail its posterior, and the top and bottom are called the dorsal and ventral sides. Most animals possess bilateral symmetry, but that does not mean that all remaining animals are radial. A few, such as sponges and some types of gastropods are asymmetrical.