This assemblage would have been more correctly styled if instead of "Vermes" it had been described as "animals unsorted." Subsequent zoologists have by degrees picked out and separated from the Vermes first one group of animals and then another. But the process is still going on, and several of the groups which are still classed under the name of "worms" might, with very great justification, be separated from each other; it is custom, rather than family resemblance, that accounts for their being retained under one heading. Widely although the various "worms" may differ from one another, one thing may be stated regarding the most of them, and that is, that they "crawl"; that is to say, they move along by means of successive contractions of successive parts of the muscular wall of their elongated bodies. This "crawling" mode of progress is the chief thing involved in the popular idea of a worm; but the popular definition of a worm includes also the larvæ of insects, such as caterpillars and beetle-grubs. The latter, it must be noted, crawl with the assistance of legs, while the true worms crawl without any such assistance. Any adornments that they may possess, whatever else they may be, are not legs. The worms were formerly included along with the insects and lobsters, in a division called Annulosa, or, Ring-bodied animals, but it has now long been recognised that the latter are worthy of a division to themselves. It will easily be seen, however, that the term Ring-bodied animals is very appropriate to all of them. If we look at either an earthworm or a lobster, we can but recognise that the body consists of a number of successive parts very similar to each other; and since the body of each is, in section, more or less round, these successive parts may very aptly be termed rings. Modern writers, however, prefer to call these parts not rings, but Metameres, _i.e._ successive parts. The symmetrical arrangement of the body in a series of such parts is called "Metamerism"; and the animals which possess it are said to be "Metameric" in structure. Sometimes also the successive parts are spoken of as "segments." Compare Fig. 12; _A_ and _C_ show the successive body-rings of worms. The earthworm, with its many rings, is one of the higher forms among the worms. Among the lowest forms there are worms in which the ring structure cannot be detected. Between the limits thus marked out, there lies, so to speak, the battleground of modern zoology. For the origin of metamerism, and the pedigree of vertebrates, are among the questions that are being discussed in connection with various groups of the worms.