The Body Structure Of Animals

 

Animals exist in a great variety of forms, from single-celled organisms to complex ones made of thousands of cells. The way they are classified, or divided into groups, depends to a large extent on how complex their bodies are. The two terms higher animal and lower animal are often used in this context. The higher an animal is, the more complex its internal organs are. In general, the distinguishing features of higher animals are segmentation, body cavities and some kind of skeleton. Segmentation The division of a body into separate areas, or segments, a step up in complexity from a simple undivided body. Generally, the more complex the animal, the less obvious its segments are. The most primitive form of segmentation is metameric segmentation, or metamerism. The segments (metameres) are very similar, if not identical. Each contains more or less identical parts of the main internal systems, which join up through the internal walls separating the segments. Such segmentation is found in most worms, for example. More complex segmentation is less obvious. In insects, for example, the body has three main parts - the head, thorax (=upper body region) and abdomen (=lower body region). Each of these is in fact a group of segments, called a tagma (pl. tagmata), but the are not divided by internal walls. They are simply external markings. Appendage A subordinate body part, i.e. one which projects from the body, such as an arm, leg, fin or wing. Arrangement of the parts: Bilateral symmetry An arrangement of body parts in which there is only one possible body division which will produce two mirror image halves. It is of almost all freely- moving animals. The same state in flowers is called zygomorphy (e.g. in snapdragons). Radial symmetry A radiating arrangement of body parts around a central axis, e.g. in starfish. In such cases, there are two or more possible body divisions (sometimes in different planes) which will produce two mirror- image halves. The same state in flowers is called actinomorphy (e.g. in buttercups).