∞ generated and posted on 2016.08.31 ∞
Multiple cells possessing the same origin, displaying similar functions, and often similarly located within an organism.
Most animals are "constructed" hierarchically with the whole organism at the top of this hierarchy and cells at the bottom. Tissues are found in this hierarchy above the level of individual cells and below the level of organs; organs in turn are constructed of more than one tissue type.
The three basic, animal primary embryonic tissues are described as ectodermal, endodermal, and mesodermal. In animals such as ourselves, the various primary embryonic tissues form the bases of muscle tissue, nervous tissue, connective tissue, and epithelial tissue.
Animals, such as ourselves, can be described as having "tissue-grade" levels of anatomical organization if their bodies consist of multiple, distinct tissues with individual tissues contributing to body function as groups of cells rather than solely or mostly as individual cells. That is, animals that possess a tissue-grade level of organization tend to be able to coordinate the functioning of multiple cells towards discrete tasks, including those involved in movement, and do this better than can animals which lack this tissue-level of body organization. The latter, particularly, are the sponges.