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.
(6). The last kind of symmetry which here needs notice is that termed serial symmetry. In the creatures which exhibit it we have a body which is not only almost always bilaterally symmetrical but which is made up of a succession of similar parts, forming a series along its main or longitudinal axis. Insects, crabs, lobsters, and other allied forms give us examples of serial symmetry, but this is perhaps best seen in such animals as thousand legs and hundred legs—millipedes and centipedes.
Besides the fundamental distinctions which depend upon the kind of symmetry governing the form of any living being, other subordinate differences exist respectively related to the conditions under which the various activities necessary for life have to be carried on. Such activities are the needful gaseous interchange, the processes of reproduction, and the acquisition of food. Thus, the most intimate relation exists between the form of the body and the manner in which locomotion has to be effected, whether by the whole body or by processes projecting from it. If the latter, then whether by paddling or jumping; if by the whole body, then whether by lateral or vertical bendings of that body.
Thus, we see that fishes, which swim by lateral flexure of the body, have the tail expanded vertically; while in porpoises, which require vertical flexions (to come rapidly to the surface to breathe), the tail is expanded horizontally. On the other hand, creatures which swim not by either kind of body flexure, but by a paddling action only, have the tail shortened, as we see in swans and turtles. Further details of this kind will be more appropriately treated of in an Essay devoted exclusively to the consideration of the forms of animals.