BRANCH VERMES:[8] THE WORMS
THE EARTHWORM (_Lumbricus_ sp.).
TECHNICAL NOTE.--Obtain live earthworms of large size, killing
some in 30% alcohol and hardening and preserving them in 80%
alcohol, and bringing others alive to the laboratory. The worms
may be found during the daytime by digging, or at night by
searching with a lantern. They often come above ground in the
daytime after a heavy rain. Live specimens may be kept in the
laboratory in flower-pots filled with soil. "They may be fed on
bits of raw meat, preferably fat, bits of onion, celery, cabbage,
etc., thrown on the soil."
=External structure= (fig. 25).--Examine the external structure of
live and dead specimens. Which is the ventral and which the dorsal
surface? Which the anterior and which the posterior end? Note the
segmented condition of the body; the number of _segments_ or
_somites_, and their relative size and shape. Note absence of
appendages such as limbs and the presence of _locomotor setæ_ (short
bristles). How many setæ are there on each segment and what is their
disposition? The _mouth_ is covered by a dorsal projection called the
_prostomium_. The _anal opening_ is situated in the posterior segment
of the body. The broad thickened ring or girdle including several
segments near the anterior end of the body is the _clitellum_, a
glandular structure which secretes the cases in which the eggs are
laid. On the ventral surface of the fourteenth and fifteenth segments
(in most species) are two pairs of small pores; two other pairs of
small openings (usually difficult to find), one between segments 9 and
10, and one between segments 10 and 11, are present. All these are the
external openings of the reproductive organs.
[Illustration: FIG. 25.--Dissection of the earthworm, _Lumbricus_ sp.]
Make drawings showing the external structure of the earthworm.
Examine a live specimen placed on moist paper or wood. Note the
characteristics of its locomotion, and the movements of its
body-parts. How do the setæ aid in locomotion?
=Internal structure= (figs. 25, 26 and 28).--TECHNICAL NOTE.--With
a fine-pointed pair of scissors make a dorsal median incision, not
too deep, behind the clitellum and cut forward as far as the first
segment. Put the specimen into dissecting-dish, carefully pin back
the edges of the cut and cover with clear water or, better, 50%
alcohol.
Note the long body-cavity divided by the thin _septa_ which have been
torn away for the most part by the pinning process. Note the thin
transparent covering of the body, the _cuticle_. Just beneath this
note a less transparent layer, the _epidermis_, and underneath this a
layer of muscles. The _muscular layer_ is made up of two clearly
recognizable sets, an outer circular layer and an inner longitudinal
layer the fibres of which are continuous with the septa.
Note, as the most conspicuous internal organ, the long _alimentary
canal_, of which a number of distinct parts may be recognized. Most
anteriorly is a muscular _pharynx_, which is followed by a narrow
_œsophagus_, leading directly into the thin-walled _crop_; next comes
the muscular _gizzard_, and next the _intestine_ which opens externally
in the terminal segment through the _anus_. The anterior end of the
alimentary canal is more or less protrusible, while the posterior
portion is held more firmly in place by the septa which act as
mesenteries. Surrounding the narrow œsophagus are the reproductive
organs, three pairs of large white bodies and two pairs of smaller sacs.
Note the _dorsal blood-vessel_ lying along the dorsal surface of the
alimentary canal, from the anterior portion of which arise several
_circumœsophageal rings_ or "hearts." These hearts are contractile and
serve to keep the blood in motion through the blood-vessels (see
later). In the most anterior of the body segments note the pear-shaped
_brain_ or _cerebral ganglion_.
TECHNICAL NOTE.--Lift carefully to right and left the reproductive
organs, thus exposing the œsophagus.
Note three pairs of bag-like structures projecting from the œsophagus.
The front pair is the _œsophageal pouches_; the next two pairs are the
_œsophageal_ or _calciferous glands_. They communicate with the
alimentary canal, and their secretion is a milky calcareous fluid.
Make a drawing that will show all the parts so far studied.
TECHNICAL NOTE.--Cut transversely through the alimentary canal in
the region of the clitellum and carefully dissect the anterior
portion of the canal away from the surrounding organs.
Note the dorsal fold of the intestine, _typhlosole_, extending into
the lumen. This fold gives a greater surface for digestion, and in it
are a great many _hepatic_ or special _digestive cells_. The entire
alimentary canal is lined with _epithelium_. Observe just beneath the
alimentary canal the _ventral blood-vessel_, and still beneath this
blood-vessel the _ventral nerve-cord_. There is a slight swelling on
the nerve-cord in each segment of the body. These swellings are the
_ganglia_. How many pairs of nerves are given off from each ganglion?
Observe in each segment, posterior to the first three or four, the
successive pairs of convoluted tubes, the _nephridia_, or organs of
excretion. Each nephridium opens internally through a ciliated funnel,
the _nephrostome_, within the body-cavity, while it opens externally
by a small excretory pore between the setæ on the ventral surface of
the segment behind that in which the nephridium chiefly lies. The
function of the nephridia is to carry off waste matter from the fluid
which fills the body-cavity.
[Illustration: FIG. 26.--Dissection to show alimentary canal in
section and nephridia of earthworm.]
Trace the ventral nerve-cord forward to its connection with the
cerebral ganglion. Note the throat nerve-ring or _circumœsophageal
collar_ connecting the ventral cord with the brain.
Make a drawing of the nervous system showing its relation to other
organs.
[Illustration: FIG. 28.--Cross-section of earthworm.]
=Life-history and habits.=--The earthworm lives in soft moist soil
which is rich in organic matter. Its food is taken into the mouth
mixed with dirt and sand. As this mixture passes through the long
alimentary canal the organic particles are taken up and digested. As
we have already seen, there are in each worm two sets of reproductive
glands, namely, male and female organs. Each earthworm produces both
egg-cells and sperm-cells, but the sperm-cells of one worm are not
used to fertilize the eggs of the individual producing them. When the
eggs are ready to be discharged from the body, the clitellum becomes
very much swollen and its glands begin an active secretion which
hardens and forms a collar-like structure about the body of the worm.
As this collar moves forward toward the anterior end of the body it
collects the eggs and also the sperm-cells previously received from
another worm, and finally slips off the head end of the animal. The
entire structure with the contained eggs and sperm-cells as it passes
off from the body becomes closed at both ends, thus forming a horny
capsule which lies in the earth until the young worms emerge. Only a
part of the eggs develop in each capsule, the rest being used as food
for the growing young. The young earthworms, though of very small
size, are fully formed before they leave the egg-capsule. Earthworms
are more or less gregarious, large numbers often being found together.
For an interesting account of the habits of earthworms see Darwin's
"The Formation of Vegetable Mold."
OTHER WORMS.
The branch Vermes comprises so large a number of kinds of animals
presenting such great differences in structure and habit that it is
impossible to give a brief statement in general or summary terms of
their external body-characters, of the structural and functional
condition of their various organs and systems of organs, and of the
course of their development and life-history as has been done for the
preceding branches. Many zoologists, indeed, do not include all the
worms or worm-like animals in one branch, but consider them to form
several distinct branches.
[Illustration: FIG. 29.--A group of marine worms: at the left a
gephyrean, _Dendrostomum cronjhelmi_, the upper right-hand one a nereid,
_Nereis_ sp., the lower right-hand one, _Polynoe brevisetosa_. (From
living specimens in a tide-pool on the Bay of Monterey, California.)]
In certain very general characters all of the animals which compose the
branch Vermes do agree. All, or nearly all, have an elongate body which
is bilaterally symmetrical, that is, which could be cut by a median
longitudinal cutting in two similar halves. In most of them also the
body is composed of a number of successive segments or somites which are
more or less alike. This kind of segmented or articulated body is also
possessed by the insects and crabs. Almost all of the worms have the
power of locomotion; usually that of crawling. For this crawling they do
not have legs composed of separate segments or joints as do the higher
articulated animals, the crabs and insects, but either have fleshy
unjointed legs, or various kinds of bristles or spines, or suckers, or
even no external organs of locomotion at all. As regards their internal
structure they have well-organized systems of organs, which show great
variety in character and degree of complexity. The special sense-organs
are usually of simple character and low degree of functional
development. Reproduction occurs both sexually and asexually; in some
species the sexes are distinct, while in others both sperm-cells and
egg-cells are produced by the same individual. Asexual reproduction is
by budding or by a kind of simple division or fission. The worms live
either in salt or fresh water, or in moist, muddy or slimy places or as
parasites in the bodies of other animals or in plants. While most worms
feed on animal substance either living or dead, some feed on living or
decaying plant matter.
=Classification.=--There is great lack of agreement among zoologists in
the matter of the classification of the worms. Not only are the various
groups which by some are called classes held by others to be distinct
branches, co-ordinate in rank with the Echinodermata, Cœlenterata, etc.,
but the limits of these groups are also constantly called in question.
It will require a great deal better knowledge of the structure and
life-history of these diverse animals before the matter of their
classification is satisfactorily settled. We shall consider briefly four
of the various groups (which we may consider as classes) which include
worms either specially familiar to us or of special interest or
importance. One or two examples of each group (the groups being selected
primarily because of the examples) will be described in some detail. By
this means we may get an idea of the extremely diverse character of the
animals which are included in the heterogeneous branch Vermes.
=Earthworms and leeches (Oligochætæ).=--The various species of
earthworms, an example of which has been studied are found in all parts
of the world; they occur in Siberia and south to the Kerguelen Islands.
They are absent from desert or arid regions, and some can live
indifferently either in soil or in water. Some near allies of the
earthworms are aquatic, living in fresh or brackish water, some in salt
water near the shore. In size earthworms vary from 1 mm. (1/25 in.) to 2
metres (2-1/6 yds.) in length. All show the distinct segmentation of the
body noticeable in the common earthworm already studied.
The leeches, some of which are familiar animals, are closely related
to the earthworms, although at first glance the similarity in
structure is not very noticeable.
TECHNICAL NOTE.--Some common water-leeches, alive or preserved in
alcohol, should be examined by the class. The animals are not
unfamiliar to boys who "go in swimming" in the small streams of
the country. The body of a leech should be examined carefully, and
drawings of it showing the external structural characters should
be made.
The body of a leech is flattened dorso-ventrally, instead of being
cylindrical as in the earthworm, and tapers at both ends. In the live
animal the body can be greatly elongated and narrowed or much
shortened and broadened. It is composed of many segments (not as many
as there are cross-lines however; each segment is transversely
annulated), and bears at each end on the ventral surface a sucker, the
one at the posterior end being the larger. These suckers enable the
leech to cling firmly to other animals. The mouth is at the front end
of the body on the ventral surface and is provided with sharp jaws.
Leeches live mostly on the blood of other animals which they suck from
the body. The common leech "fastens itself upon its victim by means of
its suckers, then cuts the skin, fastens its oral sucker over the
wound and pumps away until it has completely gorged itself with blood,
distending enormously its elastic body, when it loosens its hold and
drops off." Its biting and sucking cause very little pain, and in
olden days physicians used the leeches when they wanted to "bleed" a
person. A common European species of leech much used for this purpose
is known as the "medicinal leech." All leeches are hermaphroditic,
that is, the sexes are not distinct, but each individual produces both
sperm-cells and egg-cells. Most of the leeches lay their eggs in small
packets or cocoons. This cocoon is dropped in soil on the banks of a
pond or stream so that the young may have a moist but not too wet
environment. The young issue from the eggs in four or five weeks, but
they grow very slowly and it is several years before they attain
their full size. Leeches are long-lived animals, some being said to
live for twenty years.
=Flatworms (Platyhelminthes).=--TECHNICAL NOTE.--Collect some live
fresh-water planarians (see fig. 30), which are to be found on the
muddy bottom of most fresh-water ponds, and examine them while
alive in watch-glasses of water. Make drawings showing the
external appearance, and as much of the internal anatomy as can be
seen. The branching alimentary canal can be seen in more or less
detail, and with higher power of the microscope parts of the
nervous system can be seen also. Have also a tapeworm preserved in
alcohol or formalin to show the very flat and many-segmented body.
The flatworms include a large number of forms which vary much in shape
and habits. They are all, however, characteristically flat; in some
this condition is very marked. Some are active free-living animals, as
the planarians (figs. 30 and 31), while many live as parasites in the
alimentary canal of other animals, as do the sheep-fluke and the
tapeworms.
[Illustration: FIG. 30.--A fresh water planarian, _Planaria_ sp. (From
a living specimen.)]
The fresh-water planarians (fig. 30), which live commonly in the mud
of the bottom of ponds, are small, being less than half an inch long.
They are very thin and rather broad, tapering from in front backwards.
On the upper surface near the front they have a pair of eyes; the
mouth is on the under surface a little behind the middle of the body.
The alimentary canal is composed of three main branches, each with
numerous small side branches. One main branch runs forward from the
mouth, and the other two run backwards, one on each side of the body.
There is no anal opening, and the alimentary canal thus forms a
system of fine branches closed at the tips, and extending all through
the body. The nervous system is composed of a ganglion or brain in the
front end of the body from which two main branches extend back
throughout its whole length. From these main longitudinal branches
arise many fine lateral branches.
[Illustration: FIG. 31.--A marine planarian, _Leptoplana californica_.
(From a living specimen.)]
Of the parasitic flatworms the tapeworms are the best known. There are
numerous species of them, all of which live in the bodies of vertebrate
animals. In the adult or fully developed stage the tapeworms live in the
alimentary canal, holding on to its inner surface by hook-like clinging
organs and being nourished by the already digested food by which they
are bathed. In the young or larval stage tapeworms live in other parts
of the body of the host, and usually, indeed, in other hosts not of the
same species as the host of the adult worm.
The common tapeworm of man, _Tænia solium_ (there are several other
species of _Tænia_ which infest man, but _solium_ is the common one),
may serve as an example of the group. In the adult condition its body,
which is found attached to the inner wall of the intestine, is like a
long narrow ribbon: it may be two or three metres long. It is attached
by one end, the head, which is very small and provided with a score of
fine hooks. Behind the head the thin ribbon-like body grows wider. The
body is composed of many (about 850) joints called proglottids. There
is no mouth or alimentary canal, the liquid food being simply taken in
through the skin. Each proglottid produces both sperm-cells and
egg-cells; one by one these proglottids or joints with their supply of
fertilized eggs break off and pass from the alimentary canal with the
excreta. If now one of these escaped proglottids or the eggs from it
are eaten by a pig, the embryos issue from the eggs in the alimentary
canal of the pig, bore through the walls of the canal and lodge in the
muscles. Here they increase greatly in size and develop into a sort of
rounded sac filled with liquid. If the flesh of the pig be eaten by a
man, without its being first cooked sufficiently to kill the larval
sac-like tapeworms, these young tapeworms lodge in the alimentary
canal of the man and develop and grow into the long ribbon-like
many-jointed adult stage.
The life-history of the other tapeworms which infest the various
vertebrate animals is of this general type. There is almost always an
alternation of hosts, the larval tapeworm living in a so-called
intermediate host, and the adult in a final host. Of the domestic
animals the dog is the most frequently attacked. At least ten
different species of tapeworms have been found in the dog. The
intermediate hosts of these dog tapeworms include rabbits, sheep,
mice, etc. Some of the domestic fowl, ducks, geese and chickens, for
instance, are also infested by tapeworms, and the intermediate hosts
in these cases are usually insects or small aquatic crustaceans like
the familiar _Cyclops_.
=Roundworms (Nemathelminthes).=--TECHNICAL NOTE.--Vinegar-eels
from mouldy vinegar, and hair-worms from fresh-water pools, can
usually be readily obtained. They should be examined, and drawings
should be made of them, showing their shape and simple external
structural character. If a specimen of trichinosed pork be
obtained, the encysted stage of the _Trichina_, described in the
following account, can be shown.
The roundworms are slender, smooth, cylindrical worms pointed at both
ends. They are all very long in proportion to their diameter, although
their actual length may be short. Some species are of microscopic
size; as the _Trichina_ worm, which is about 1/20 in. long; while the
guinea-worm, one of the worst parasites of man, may reach a length of
six feet. Many of the roundworms are parasites living in the various
organs of other animals. Some, however, lead an independent free life
in water or in damp earth.
[Illustration: FIG. 32.--A vinegar eel, _Anguillula_ sp. (From a
living specimen.)]
Familiar examples of roundworms are the so-called vinegar-eels
(_Anguillula_) (fig. 32) to be found in weak vinegar, and other
species of this same genus which live in water or moist ground or in
the tissues of plants, doing much injury. The hair-worms (_Gordius_)
or horse-hair snakes, which are believed by some people, to be
horse-hairs dropped into water and turned into these animals, are also
familiar examples of roundworms. They are often found abundantly in
little pools after a rain, and it is sometimes said that these worms
come down with the rain. They have in reality come from the bodies of
insects in which they pass their young or larval stages as parasites.
The hair-worms all live as parasites during their larval stage, and as
free independent animals in their adult stage. Some of them require
two distinct hosts for the completion of their larval life, living for
a while in the body of one, and later in the body of another. The
first host is usually a kind of insect which is eaten by the second
host. The eggs are deposited by the free adult female in slender
strings twisted around the stems of water-plants. The young hair-worm
on hatching sinks to the bottom of the pond, where it moves about
hunting for a host in which to take up its abode.
[Illustration: FIG. 33.--_Trichina spiralis_, encysted in muscle of a
pig. (From specimen.)]
The terrible _Trichina spiralis_ (fig. 33), which produces the disease
called trichinosis, is another roundworm of which much is heard. This
is a very small worm which in its adult condition lives in the
intestine of man as well as in the pig and other mammals. The young,
which are borne alive, burrow through the walls of the intestine, and
are either carried by the blood, or force their way, all over the
body, lodging usually in muscles. Here they form for themselves little
cells or cysts in which they lie. The forming of these thousands of
tiny cysts injures the muscles and causes great pain, sometimes
death, to the host. Such infested muscle or flesh is said to be
"trichinosed," and the flesh of a trichinosed human subject has been
estimated to contain 100,000,000 encysted worms. To complete the
development of the encysted and sexless _Trichinæ_ the infested flesh
of the host must be eaten by another animal in which the worm can
live, e.g. the flesh of man by a pig or rat, and that of a pig by man.
In such a case the cysts are dissolved by the digestive juices, the
worms escape, develop reproductive organs and produce young, which
then migrate into the muscles and induce trichinosis as before. But
however badly trichinosed a piece of pork may be, thorough cooking of
it will kill the encysted _Trichinæ_, so that it may then be eaten
with impunity. Some people, however, are accustomed to eat ham, which
is simply smoked pork, without cooking it, and in such cases there is
always great danger of trichinosis.
=Wheel animalcules (Rotifera).=--TECHNICAL NOTE.--Live specimens of
Rotifers can be found in almost any stagnant water. Examine a drop
of such water with the compound microscope, and find in it a few
small, active, transparent creatures, larger than the _Paramœcium_
and other Protozoa in the water and which have the appearance shown
in fig. 34. They may be known by the constant whirling, or rather
vibrating, circlet or wheel of cilia at the larger or head end of
the body. These wheel animalcules may be studied alive by the class.
Although usually darting about, the animalcules occasionally cease
to move, when, because of their transparency, almost the whole of
their anatomy can be made out. Their feeding habits can also be
readily observed, and the food itself watched as it moves through
the body. Make drawings showing as much of the anatomy as can be
worked out. Note especially the "mastax" or gizzard-like masticating
apparatus in the alimentary canal.
The wheel animalcules (fig. 34) or Rotifers look little like the other
worms we have studied. But they are nevertheless more nearly related
to the worms than to any other branch of animals. They are all small,
about 1/3 mm. long, and have a compact body. They are aquatic and feed
on smaller animals and plants or on bits of organic matter which they
capture by means of the currents produced by the vibrating cilia of
the "wheel." Small as they are they have a complex body-structure,
with well-organized systems of organs. For a long time, however, they
were classed by naturalists with the Protozoa on account of their
size. They are found all over the world, mostly in fresh water; a few
are marine. More than 700 species of them are known.
[Illustration: FIG. 34.--A wheel animalcule, _Rotifer_ sp. (From
living specimen, Stanford University.)]
An interesting thing about the Rotifers is their remarkable power to
withstand drying-up. When the water in a pond or ditch evaporates some
of the Rotifers do not die, but simply dry up and lie in the dust,
shrivelled and apparently lifeless, yet really in a state of suspended
animation. On being put into water they will gradually fill out to their
full size and shape, and finally resume all their normal activities. In
this dried-up condition Rotifers may persist for a long time, several
years even, although otherwise their natural life is short, being
probably of not over two weeks' duration. Certain other of the lower
animals have this same power of withstanding desiccation.
FOOTNOTE:
[8] The author recognizes the untenability of the group Vermes as a
group co-ordinate with the other branches of the animal kingdom, and
that "Vermes" has been discarded in modern text-books. But because of
the very scant consideration which can be given the various kinds of
worm-like animals the course of the older text-books will be followed,
and all of the worm-like animals, as far as referred to in this book,
be considered under the group name Vermes.