Stories of the Universe: Animal Life Part 4

CHAPTER VII

WORMS

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

This a.s.semblage 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 cla.s.sed 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 larvae of insects, such as caterpillars and beetle-grubs. The latter, it must be noted, crawl with the a.s.sistance of legs, while the true worms crawl without any such a.s.sistance. 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 cla.s.s 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 ca.n.a.l, 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--_Linnaea 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 Chaetopods (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 Oligochaeta (Few-Bristles), which include the familiar earthworms; and the Polychaeta (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 ill.u.s.tration 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 l.u.s.tre (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 Syllidae 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.

[Ill.u.s.tration: 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.]

The tube-dwelling worms are represented in our picture by _Terebella_, _Serpula_ and _Spirorbis_, all very common forms on the English coasts.

The _Terebella_ glues around its body a number of grains of sand and bits of sh.e.l.l, thus forming a case; the projecting threads at the head end are the gill-filaments, borne by the anterior segments of the body.

These are plumed; the thread-like structures which are seen to lie in front of them are the tentacles or feelers. _D_, _Serpula_, is common on sh.e.l.ls and stones. The animal has a plumy bunch of gill-filaments, brilliantly coloured, and a stopper with which it can close the mouth of its tube. This precaution is necessary to keep out its predatory cousins belonging to the Errantia, who poke in their heads and eat the tube-dwelling worms. _E_ is _Spirorbis_, a minute form with a coiled tube, which looks at first sight like a small univalve sh.e.l.l. It is common everywhere, on sh.e.l.ls and stones, and encrusting Fuci and other seaweeds, which it sometimes covers almost completely. Spirorbis also has plume-like gills and a stopper. In the latter is a cavity where the creature's eggs are incubated for a time.

The reader will have no difficulty in finding and identifying both _Serpula_ and _Spirorbis_. _Terebella_ is frequently washed up on a sandy sh.o.r.e. On the Lancas.h.i.+re coast one may feel sure of finding this and many other sand-dwelling animals, after an east wind. The east wind, driving back the water at low tide, kills these creatures with cold, and presently they are washed up dead or dying by the high tide.

_Pectinaria_, another worm with a tube of sand-grains, in which, however, the body lies loosely within the tube, may also be found in thousands under the same circ.u.mstances.

TABLE SHOWING THE CLa.s.sIFICATION OF VERMES OR WORMS

=Grade III.= { { WITH MESODERM, { PLATYHELMINTHES, { _A._ TURBELLARIA or PLANARIANS.

BUT WITHOUT { or FLAT-WORMS. { _B._ CESTODA or TAPE-WORMS.

BODY-CAVITY. { { _C._ TREMATODA or FLUKE-WORMS.

=Grade IV.= { NEMERTINES.

WITH MESODERM { NEMATODA or THREAD-WORMS.

AND { HIRUDINIA BODY-CAVITY. { or LEECHES. { _A._ ARCHIANNELIDS.

{ { { CHaeTOPODA. { _B._ OLIGOCHaeTA.

{ { _C._ POLYCHaeTA. { (_a_) Tubieola.

{ (_b_) Errantia.

We must not forget to say something regarding the most commonly known member of the Vermes, the familiar earthworm. The worms are the first of the great group of animal life in which we find true land animals. There are terrestrial forms among the lowest worms, at least forms that live in earth that is damp; but the earthworm is in the strictest sense a terrestrial animal. Darwin showed that it not only dwells in the soil, but is in a sense the manufacturer of soil, since the fertility of the earth depends greatly upon the work of earthworms. They pa.s.s the soil through their bodies, digesting the organic particles they find in it, and thereby loosen the soil, reduce it to a state of fine division, and render it more fit to support the growth of plants. The "worm-casts"

formed by the soil that the earthworm has pa.s.sed through its body may not have been noticed by everybody. More obvious are the worm-casts in sand left by the sand-dwelling marine annelids. These everyone must have seen who has walked on a sandy sh.o.r.e at low tide.

The worms include many puzzling forms, which have not been alluded to here. Among these must not be forgotten the Rotifers, or wheel-bearing animals. These are of minute size, and when first discovered were therefore placed amongst the Infusoria. They are common in ponds.

CHAPTER VIII

ARTHROPODA, THE LOBSTERS, SPIDERS AND INSECTS

The above is a very descriptive name for a division which includes the Crabs and Lobsters and the Insects. Formerly they were included, along with the worms, under the name Annulosa, the Ringed Animals. They resemble these as possessing what is termed metameric symmetry, but they are distinguished from them as the Leggy Animals, a fact which is explained in the name, Arthropoda, joint-footed. Worms, as we have seen, have no true legs, but the Arthropods, theoretically, have a pair of legs to every ring. In some of the lower members of the group this is literally the case, the Centipedes, or hundred-footed animals, for example (Fig. 13). In higher forms the number of legs is greatly reduced; several successive rings may become merged with one another, losing, along with their independence, their legs. The true Insects, thus, have only three pairs of legs and the Spiders four.

[Ill.u.s.tration: FIG. 13.--A Centipede, _Lithobius elongatus_, from Tunis, slightly reduced in size.]

What are theoretically regarded as legs, however, may practically be turned to many other uses, according to the position of the particular body-ring to which they are attached. Thus, in the case of a body-ring near the mouth, we find such things as "jaw-feet," maxillipedes--that is to say, legs used for jaws. It consequently results that zoologists are sometimes driven to speak of "walking legs," or, hiding the tautology under a Latin phrase, "ambulatory legs"; and absurd although this may seem, it is sometimes quite necessary for the sake of accuracy. It is therefore more convenient to speak of the "appendages" of a body-ring than of its legs. For this vague term can be applied equally to all the row, whatever their uses. Among the different forms taken by the "appendages" are those of "antennae," long, hair-like feelers attached to the head; "chelae," or claws, such as the large claws of the lobster; "chelicerae," or "claw-horns," tearing appendages attached to the head; "mandibles," mouth appendages used for biting, etc., etc. The reader who wishes to attain a clear idea of the structure of a segmented animal, and of the ways in which its parts are modified, should consult Huxley's cla.s.sical study of "The Crayfish" (International Science Series).

The Arthropoda include two main groups--the Crustacea, or Jointed Animals of the water, which breathe by gills; the Insects, or Jointed Animals of the land, which breathe through tubes in their sides, called tracheae.

The Crustacea include the familiar Crabs and Lobsters. These are among their highest forms as well as their largest, and if we begin at the beginning we must seek much smaller forms. The group called Entomostraca include the so-called Freshwater Flea, a very active little thing found in English ditches, and a great many other freshwater forms: also the little Cypris, which has a s.h.i.+eld forming a sort of bivalve-sh.e.l.l, and is interesting from its wide occurrence as a fossil form. Most of the Entomostraca have a larval form called a Nauplius; but this larva refuses to tell us anything about the past history of the Arthropods. It is itself already a jointed animal with legs. So we see that the Arthropods, unlike the worms and the Chordata, have obliterated all record of their poor relations. The parasitic "fish-lice," so-called, are entomostracous Crustacea, often greatly degenerated in consequence of their habit of life. Some live in the gill-chambers of a fish, some on, or even embedded in the skin.

[Ill.u.s.tration: FIG. 14.--Sh.e.l.l of the Bell Barnacle, _Bala.n.u.s tintinnabulum_, one-half the natural size. The figure shows several successive generations, perched one upon another.]

Among the most curiously modified forms of the Crustacea are the Barnacles or Cirripedia. These creatures, like the sponges, have a free-swimming larvae, which eventually fixes itself by its anterior end, so that the adult animal pa.s.ses its existence upside down. The young is an ordinary little creature with jointed legs, but the adult protects itself by a strange armour of sh.e.l.l. An intermediate stage exists in which the creature eats no food; it has therefore been compared with the chrysalis of insects. At the top of the adult sh.e.l.l two little valves open and shut, allowing the legs to dart out and seize upon prey. These legs, gathered into a bunch, and extended and retracted together, remind one of the fingers of a hand opening and closing. They are clothed with a fringe of "cirrhi" or small processes; hence the name of the group.

The Common Barnacle of our own sh.o.r.es, sometimes called the Acorn-Sh.e.l.l, is found on sh.e.l.ls and stones, and often on those that are left uncovered between tides. In places where the rocks of the coast are very steep, a belt of white, several feet or yards deep, may often be seen above low-water mark. This white zone, when examined more nearly, is found to consist of barnacles, so crowded together that they obscure the natural colour of the rock. The Common Barnacle is one of the smaller species of the genus: in warmer seas barnacles attain to a much greater size (Figs. 14 and 15).

[Ill.u.s.tration: FIG. 15.--Sh.e.l.ls of a Barnacle, _Bala.n.u.s hameri_, found in European and North American Seas, natural size.]

The higher Crustacea, Malacostraca, include the familiar Crabs and Lobsters, Decapoda. The lobsters receive the name of Macrura or Big-tails; a.s.sociated with them are the Shrimps and the Hermit-Crabs (Fig. 16). The latter are therefore not crabs at all, but somewhat divergent lobsters. Their tails are soft, and they thus require protection: they choose the dried sh.e.l.l of some univalve mollusc and live in it (Fig. 16). How far the case is that they need a house because their tails are soft, and how far the contrary is true that their tails are soft because they live in a house, it would be difficult to say.

Readers of another volume in this series, Professor Hickson's "Story of Animal Life in the Sea," will remember that the hermit-crab often offers a curious instance of "commensalism" or partners.h.i.+p with other animals.

The hermit-crab was, in fact, one of the earliest instances in which such a partners.h.i.+p was observed, the companion being in this case a sea-anemone perched on the sh.e.l.l in which the crab lives.

[Ill.u.s.tration: FIG. 16.--Hermit Crabs. _A_, _Aniculus typicus_, from the Indo-Pacific Seas, one-half of the natural size. _B_, _Caternus tibicen_, from the Indo Pacific Seas, slightly enlarged.]

The true Crabs are called Brachyura, or Short-tails; for obvious reasons, the tail of a crab being very curiously modified and tucked in under the carapace or "sh.e.l.l." A form exceptional in the fact that frequents the land is the Land-Crab of the West Indies (Fig. 17).

Another land crustacean, _Birgus latro_, the Robber Crab, belongs to the previous group.

[Ill.u.s.tration: FIG. 17.--Land Crab, _Gecarcinus ruricola_, from the West Indies, one-half of the natural size.]

In addition to the above the Malacostraca include the Arthrostraca, or crustaceans which have the front of the body jointed as well as the tail, so that there is no large s.h.i.+eld formed by the fused armour of several segments (cephalo-thoracic s.h.i.+eld, _cf._ Figs. 16 and 17), as in crabs and lobsters. The Amphipoda, or Sand-hoppers, sometimes called Sand-fleas, are familiar examples of these. There are several common kinds found on our English sh.o.r.es, and sometimes they appear in such numbers, hopping above sand or seaweed left by the tide, that they seem to form a sort of cloud, every unit of which, however, is but in the air an instant, falling and giving place to some other, while it prepares for a fresh hop. The so-called Freshwater Shrimp, _Gammarus_, is another common member of the Amphipoda. Fig. 18 shows the general form of a Sand-hopper. Nearly allied are the Isopoda or Wood-lice, interesting because they are among the few terrestrial forms of the crustacea; they live, however, in damp places, and are but too well-known in gardens, where the gardener often mis-names them "insects."

[Ill.u.s.tration: FIG. 18.--A Sand-hopper, _Pallasea Cancellus_, from Siberia, natural size.]

[Ill.u.s.tration: FIG. 19.--A South American Spider, _Ctenus ferus_, from the Amazon region, natural size.]

The mention of terrestrial forms would naturally bring us to the discussion of the true Insects. In the Arthropoda we for the first time meet with terrestrial animals except in scattered instances, and the true Insects are the largest and most important group of these. There are, however, various creatures belonging to the Arthropoda which are neither Crustacea nor yet Insects. Among these is the familiar spider, an "insect" in popular language, but not so described by the zoologist.