When the great naturalist, LinnÆus, framed his classification of the animal kingdom, he included in the division Vermes or Worms, nearly everything except the vertebrates and insects.

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.

Among the lowest forms of worms are the Planarian worms, already alluded to as examples of the third grade of animal existence. These belong to the class Turbellaria, which is represented by plenty of both fresh water and marine forms in our own country and on its coast. The Turbellaria are divided into groups called Acoela, Dendrocoela, and Rhabdocoela. These names allude to the intestine, which in the first group is wanting, in the second branched like a tree, and in the third straight. The Cestoda or tape-worms, which absorb nourishment through the skin, and therefore need no alimentary canal, and possess none; and the Trematodes, represented by the Liver-fluke, which infests sheep, together make up the group of flat-worms (Platyhelminthes), of which mention has already been made (p. 44). In all of them the body is more or less flat, and the digestive cavity, like that of Coelenterates, has but one opening, the mouth. The life-history of parasitic worms is described in a well-known volume by Leuckart, which forms the basis of our knowledge on the subject. Since its publication, discoveries regarding parasites have been constantly added by other observers.

The history of the Liver-fluke is a most complicated example of alternation of generations. The adult form infests the sheep's liver. There it produces eggs, which afterwards find their way into water. Here they die unless they find their way into a certain water-snail, which many of them do. Within this snail—LinnÆa truncatula—the egg develops into a sac-like body, called a sporocyst. This produces within itself numbers of a small creature which is called the Redia form. These in turn produce a tailed form, called a Cercaria, which gets out of the snail, swims in water, and finally settles down on some plant. Here it is eaten by an unfortunate sheep, within which it develops into the adult fluke.

The other great divisions of the Vermes are as follows: The Nematodes or thread-worms, a group of parasites which includes the dreaded Trichina; the Nemertines, a group mostly carnivorous, possessing a curious proboscis, and often an armed skin; the Leeches or Hirudinea, and finally, the ChÆtopods (Bristle-footed Worms), the highest group of all, containing the forms often spoken of as Annelides—i.e. Ring-shaped Worms. These last are again subdivided into the following: The Archiannelida or Primitive Annelids, some of which have a curious ciliated larva, already referred to (p. 42) as the typical Trochosphere or Wheel-ball; the OligochÆta (Few-Bristles), which include the familiar earthworms; and the PolychÆta (Many-Bristles). Of the latter, some, the Tubicola, live in tubes which may or may not be fixed to some object; while others, the Errantia, or Wanderers, are free and very active. Nereis, the Rainbow Worm (p. 159) may be named as an example. Our illustration shows instances of each group. A is the Sea-Mouse, a bristly creature so named by some very imaginative person. It has two kinds of bristles, long and short, the former being possessed of a peculiar lustre (see p. 73). C is Syllis, one of a very curious family of worms. In both A and C are seen a row of paired appendages; these are not "legs," but expansions called "parapodia" which serve the purpose of legs, besides which they frequently act as breathing organs, a special part being appropriated to this purpose. Each of these animals is active and carnivorous, and has a head. The SyllidÆ are remarkable for the very peculiar way in which they divide, new individuals being formed and cast off from the end of the body. There is, however, a deep-sea form of Syllis which divides in a very odd manner, giving rise to new individuals placed transversely. The result is a most extraordinary looking creature, a network of worms with numerous heads, each branch being eventually provided with one of its own.

Fig. 12.—Worms. A, a Sea-Mouse, Aphrodite aculeata; B, Terebella littoralis; C, Syllis; D, Serpula vermicularis; E, Spirorbis nautiloides, on a piece of seaweed.]