Immunology is one of the Biology Topics focused on understanding the immune system and its response to pathogens and diseases.
Animal Kingdom – Animalia Phylum, Subphylum
Animals are those organisms that are eukaryotic, multicellular, and heterotrophic in their mode of nutrition. Animal cells do not have cell walls. Except for a few, most animals are mobile. Multicellular animals are often called metazoa which have been divided into two branches: Parazoa and Eumetazoa. In the flow chart, animals are arranged progressively from simple parazoa to highly complex animals.
In Parazoa, the animal body is formed of loosely aggregated cells. These animals have poorly differentiated tissues and have no organ or digestive cavity (e.g., sponges). However, in Eumetazoa, the cells are properly arranged into tissues and organs. Their digestive tract is the mouth but with or without an anus.
The Eumetazoa is further classified on the basis of (a) the number of germ layers present in the embryo; (b) the symmetry of the body of the organism; and (c) the mode of origin of mouth. Those animals which have two germ layers in the embryo are called diploblastic animals and those having three germ layers are called triploblastic animals. In diploblastic animals, the outer cell layer is called an ectoderm, while the inner layer is termed an endoderm; and both of these germ layers enclose a non-cellular jelly-like layer, the mesoglea. The triploblastic animals have three germ layers, namely ectoderm, mesoderm, and endoderm, in their bodies.
Most animals including humans have symmetrical right and left sides. In other words, the two sides of the body are mirror images of one another. Such body symmetry type is called bilateral symmetry. Some animals have radial body symmetry which can be defined as an arrangement of usually similar parts in a regular pattern around a central axis.
On the basis of mouth origin two types of animals have been recognized: Protostomia (mouth arising from or near the blastopore of gastrula; gastrula is an embryonic stage) and Deuterostomia (mouth arising anteriorly at some distance from the blastopore).
Further, based on the presence or absence of body cavities or coelom, animals are grouped into acoelomate, pseudocoelomata, and coelomate. The acoelomates do not have a body cavity, because in them the space between the body wall and digestive tract is filled with parenchymatous tissue. Although the pseudocoelomates have a cavity between the body wall and digestive tract, but this cavity does not arise from the mesoderm and is not lined by epithelial cells. The coelomates have a true body cavity or coelom which originates from the mesoderm and is lined by epithelial cells of mesodermal origin.