WORMS
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.
[Illustration: 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 shell, 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 shells 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 shell. It is
common everywhere, on shells 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 shore. On the Lancashire 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 circumstances.
TABLE SHOWING THE CLASSIFICATION 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.
{ {
{ CHÆTOPODA. { _B._ OLIGOCHÆTA.
{
{ _C._ POLYCHÆTA. { (_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 pass 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 passed 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 shore 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.