(3). The next morphological category may be spoken of as _unipolar symmetry_. Bodies which exemplify it are like those included in the last category, save that the two poles of the body are not alike. Instances of this symmetry are to be sought in creatures which have one end of their body fixed, or which always or mostly move with the same end of the body in front, and thus have their two extremities in more or less constantly different relations to surrounding influences. The lowest worms and sponges may serve as examples of this symmetry in its simplest expression. As also may the curious compound tunicary called _Pyrosoma_.[68] In all such creatures the body does not extend out in the form of lateral prolongations. But in many others it does send out processes on all sides, and in various directions, as in most trees and all plants which have a definite axis of growth, so that unipolar symmetry is the predominant symmetry in the vegetable kingdom. (4). But unipolar symmetry with diverging outgrowths leads us to the next category which may be called _radial symmetry_. Under this head are included the forms of such creatures as possess unipolar bodies from which equal and corresponding outgrowths radiate in different directions. We have examples of this in the starfishes, in the sea anemones, and in such plants as the melon cactus. But the outgrowths may project in only four directions, each being at right angles with the two neighbouring outgrowths. We thus get a crucial form of radiation, in which the body may be described as having one main axis (in the direction of motion) crossed by two other shorter but equal axes at right angles to it and to each other. We have an example of this in _Tetraplatia volitans_,[69] an aquatic creature with an elongated body, which presents four distinguishable longitudinal surfaces, of which each opposite and corresponding pair is hardly distinguishable from one another. (5). This form leads us directly to that kind of symmetry which is predominant in the animal kingdom and which is called _bilateral symmetry_. Forms of this kind exhibit four aspects which may be distinguished as right and left, dorsal and ventral. The body here presents a long axis (in the direction of motion) crossed by two shorter axes at right angles to it and to each other. Of these shorter axes, one connects the dorsal and ventral surfaces, while the other connects the lateral (right and left) surfaces, and these two axes may be, and generally are, unequal. All worms, insects, mollusks, fishes, birds, reptiles, and beasts, are examples of creatures with bilateral symmetry. The dorsal and ventral aspects of the body generally differ in correspondence with the different relations to surrounding conditions which they usually bear, as notably in snakes and creatures which glide with their bellies applied to the surface of the ground.