(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. There are a multitude of aquatic creatures which cannot be properly spoken of as either "fixed" or "mobile," for they are in fact both. They are creatures which move about by the help of others, being themselves fixed to other creatures which are actively locomotive. Thus, sea-snails, lobsters, fishes, whales, and even ships, bear about with them sometimes lowly-organized plants; but often other animals, permanently fixed to and growing parasitically upon them and having the shape of their body suited to their peculiar situation. Often such parasites form flattened encrustations on their involuntary hosts--as is the case with the acorn shells or sessile barnacles.[70] Others have elongated bodies, which stream through the water with the motions of the creatures carrying them. We see this in confervoid growths, also in ordinary barnacles, and in certain modified crab-like creatures, such as _Lerneocera_.[71]