ANIMAL TISSUE, ORGAN AND ORGAN SYSTEM
There is only one cell in the body of the animals of the phylum Protozoa. Animals of the Phylum Porifera are simplest multi-cellular animals. Their cells do not form actual tissue. Though tissues, organs and systems are not formed in members of the phylum Cnidaria (previously known as Coelenterata), their body cells are, however, arranged in two layers.
In the animals from phylum Platyhelminthes to Chordata, tissues, organs and organ systems are seen. Members of those phyla are three layered. From there three embryonic cell layers (namely ectoderm, mesoderm and endoderm), tissues, organs and systems are formed.
Tissue: In the body of multi-cellular animals some cells remain together andperforma particular function. These cells are either formed from the ectoderm, mesoderm or endoderm of the embryo and remain close together. Inter-cellular materials or matrix are present between these cells. These cells may be similar or of different types. Arising from the embryonic cell layer when certain types of cells when remain in a particular place of the animals' body and collectively perform a common function, those cells and the inter-cellular materials or matrix secreted by them are collectively called the Tissue. That is to say, cells of a particular tissue are similar in its, origin, function and structure. The subject in which the different types of tissue are discussed is called Histology.
A. Tissue: Originating from a same place when more than one cell with similar structure and function performs a common function, then those groups of cells are called Tissue. For example, blood is a kind 'of fluid connective tissue. In the liquid matrix or plasma of blood there are red corpuscles, white corpuscles and platelets. Blood is developed from the mesoderm of the embryo.
B. Cell: Cell is the structural and functional unit of tissue. For example, Red blood corpuscles (Erythrocytes), White blood corpuscles (Leucocytes) and Platelets (Thrombocytes) are various types of blood cells. Of these the redcourpuscles transport oxygen and carb.on dioxide, white corpuscles take part in the defense of the body and the platelets take part in blood clotting at the site of injury and stop unwanted blood loss.
Blood cells and the plasma together form the blood tissue.
Blood generally takes part in the internal transportation of the body.