PARASITES FREE WHILE YOUNG.
We have brought together in the former chapter the animals which live
at the expense of their neighbours, without seeking for anything
except shelter. They seize their prey as they pass, are nourished by
the blood of their neighbours, but never think of establishing
themselves in their organs during any period of their life. They are
almost as much carnivora as parasites, and only differ from the former
class because they spare the life of their victims. They are unlike
ordinary parasites, since they are contented with their food alone;
and their appearance from the period of their entrance into the world
is that of free animals. Those whose history we are now about to
sketch, live in freedom like the preceding during all the time that
they are young; like them, they are completely independent during the
first period of their life; but when they have arrived at mature age,
when the endless cares entailed by their young ones come upon them,
they change their costume and accommodate themselves as well as they
can to the new lodging which they have chosen. There is often not the
least resemblance between these creatures in their youth and their
adult state. All these parasites have lived a joyous life before 139
choosing the host which is to serve them as a cell; but though in many
species we see both sexes shut themselves up as in a cloister, some
species are to be found in which the female alone seeks for extraneous
aid; which is not surprising, since she alone undertakes all the
charge of the family, and this would be beyond her strength, and would
endanger the life of her offspring, if she did not receive help and
protection.
The host resembles in some respects a lying-in hospital, especially
when the female alone seeks for herself a resting-place and her food,
which is not always the case. We find, in fact, in a considerable
number of Lernæans, that the microscopic male passes unperceived upon
his female, and when he renounces his bachelor life, she feeds him
with her own blood. There cannot be a more faithful husband, since he
only plays the part of a spermatophore. We find a still more curious
example in this respect, and in which the dignity of the male is not
less compromised; we refer to the Bonelliæ which live freely in the
sand, and whose males establish themselves parasitically on the sexual
organs of the female. She herself lives by her own industry, nourishes
her husband, and alone provides for all the requirements of maternity.
In a later part of this work, we shall mention worms which live in
freedom in damp earth, and whose direct progeny, entirely composed of
females and hermaphrodites, can only exist as parasites. These worms
do not resemble their mother but their grandmother, and if their
descent had not been traced, they would doubtless have been taken for
species entirely distinct from each other. Thus it is not always the
whole family which is modified; the male often preserves all the 140
attributes of his sex and of his youth, while the female changes
entirely her appearance and her mode of motion, especially at the
approach of the period when the interest of the species prevails over
that of the individual.
We can nowhere find more graceful and regular forms during the whole
of their early youth than those of many of these parasites; we can
never see more ungraceful, we might almost say more comical, attitudes
than those of the greater part of these creatures when full grown. One
might take them for some misshapen excrescence, or some scrap of
wasted flesh on the body of their host. A certain number of insects
are found which lead this singular kind of life, but this is more
especially the case among the crustaceans, particularly the copepod
crustaceans. Among all these we find the most absurd recurrent forms;
in fact these animals instead of carrying on their evolution, like the
caterpillar which becomes a butterfly, retrograde rather than advance,
and acquire an appearance and character which prevent us from
recognizing their origin. Many of these are at present known, whose
graceful form is so completely changed, that without referring to the
study of their embryo state, one could not tell to what class they
belong. Nothing remains of their organs except the sexual apparatus
and a shapeless skin. These curious parasites live also on the surface
of bodies, and sometimes in the cavity of the mouth; but in fishes
they are most frequently found in the branchial membranes. They look
like natural setons, and it is not impossible that they sometimes
fulfil the same functions.
We will first examine some insects, then certain isopode crustaceans, 141
an order to which the Cloportidæ (wood-lice) belong, many of which
require uninterrupted assistance; then we will turn to the Lernæans,
which surpass all the rest in their many and bizarre transformations.
[Illustration: Fig. 26.--Male Chigoe.]
[Illustration: Fig. 27.--Head of Chigoe.]
[Illustration: Fig. 28.--Female Chigoe.]
We have first to speak of the Chigoe, an insect, the female of which
alone demands lodging and provisions, the male being contented, like
those of the preceding chapter, with pillaging his victim as he passes
by. This parasite of man inhabits South America, and has received the
name of _Pulex penetrans_, or, according to the latest nomenclature,
of _Rhyncoprion penetrans_. It is a very small species, which pierces
the shoes and the clothes with its pointed beak (Fig. 27), and
penetrates into the substance of the skin; the male (Fig. 26) is
contented with sucking the blood, and then resumes its wanderings,
like the parasites of which we have spoken in the preceding chapter;
while the female finds for herself a hiding-place, and becomes of such 142
a monstrous size that the entire insect is nothing more than an
appendage of the abdomen, as may be seen in the annexed figure. This
insect is well known, since it attacks man, and usually establishes
itself on his toes, but it occasionally fixes itself in the same
manner on the dog, the cat, the pig, the horse, and the goat. It has
also been seen upon the mule. Mons. Guyon has paid much attention to
it, but we owe the last observations to Mons. Bonnet, a French navy
surgeon, who passed three years in Guiana, and has ascertained that
the chigoe fortunately does not extend beyond the 29th degree of south
latitude. Another parasite, well known by sportsmen, is the tick. It
is not an insect like the flea, but an arachnid, a kind of acarus,
which passes through its last stages of development under the skin of
a mammal. It is called _Ixodes ricinus_, and Professor Pachenstecher
has carefully studied its organization. The ticks especially attack
dogs, but are also found on the roebuck, the sheep, the hedgehog, and
even on bats.
Some years ago it was propagated in an extraordinary manner on
roebucks in the woods of the Duke of Arenburg, in the environs of
Louvain. They are sometimes found also on man. We know of two
instances: the first is that of a lady at Antwerp, who had a small
tumour on her shoulder, which was removed, and enclosed a living tick.
Leeuwenhoek gives an instance of a woman of the lower classes who had
a tick in the middle of her stomach. Moquin-Tandon relates that
Raspail found some on the head of a little girl four or five years
old. He also gives an instance of a young man who, returning from
hunting, found a tick under his arm; and while on the site of a 143
sheep market, a servant found one morning three attached to the skin
of his breast. Delegorgue speaks of some very small reddish ticks in
Africa, which cover the clothes by thousands, and produce distressing
itching. Others are found in different parts of the globe, and
twenty-four species have been described. Several new American Ixodes
have been noticed lately by Mr. Packard on the stag, the monax marmot,
the _Lepus palustris_, &c. These arachnida live at first in freedom in
the bushes, but after fecundation the female attacks the first mammal
which she finds in her way, and establishes herself upon it; dogs
become infested with it by running in and out among the brushwood.
The _Argas reflexus_ lives on pigeons, and is allied to the _Ixodes_.
R. Buchholz has lately studied many new acaridæ found on different
birds.
If the forms are not so varied among the isopods as elsewhere, many
among them present nevertheless the most extraordinary appearance, the
most unexpected contour. Most of the parasitic isopods instal
themselves in the thoracic cavity under the carapace of a neighbour,
and make themselves contented in the small space which remains to
them. After having disposed of their luggage, they arrange themselves
scrupulously according to the extent of the lodging which they occupy,
and, rather than interfere with the branchiæ, they raise up the walls
of the cephalothorax, thus forming a sort of tumour which betrays the
presence of the intruder. Others are found which are not contented
with a natural cavity; they raise the scale of the skin of a fish,
perforate or hollow out the true skin, or even pierce through the
walls of the abdomen, in order to establish themselves in the 144
intestines, still keeping up a communication with the exterior. A very
common species of this class is called _Bopyrus_. We often see
beautiful prawns, which are usually remarkable for their fine rose
colour, exposed for sale in shop windows. If we examine them at
certain seasons, especially in France, we perceive that the carapace
at the side is raised; and if we take it off with some precaution, we
discover underneath an irregular flattened body, which fishermen take
for a young sole on account of its shape. This is the female bopyrus.
The many appendages of the thorax, the division into rings, the
symmetry of the body, all have disappeared, and the claws, the traces
of which are scarcely seen, are no longer similar on the right and
left sides. The male remains small and independent, and preserves the
livery of the order to which he belongs. On the coast of Labrador, a
bopyrus behaves in the same manner towards a Mysis. We have found
under the carapace of a pagurus a female bopyrus full of eggs, so much
flattened that it might have been taken for a leaf accidentally
introduced into this cavity.
Fritz Müller has divided the Bopyridæ in the following manner:--
1. Those which fix themselves on the appendages or in the branchial
cavity of decapods; these are the Bopyri, Iones, Phryxi, Gyges,
Athelgi, &c.
2. Those which live in the thoracic cavity of some Brachyuri, as the
_Entoniscus_.
3. Those which live in the cirrhipeds, like the _Cryptoniscus_, as
well as the _Liriopes_.
4. Those which live on copepods as true parasites, as the
_Microniscus_ (_M. Fuscus_).
The _Iones thoracicus_, the _Cepes distortus_, the _Gyges 145
branchialis_, and so many others live, like the Bopyri, in the
thoracic cavity of different decapod crustaceans, and the females
throw off at the same time their organs of sense and all their fishing
and travelling apparatus.
Rathke, a learned professor of Königsberg, was the first to notice an
isopod, known under the name of _Phryxus paguri_, which lives on the
stomach of a pagurus, attached to it by its back, so that the stomach
of the parasite is turned, like that of the pagurus, towards the
partitions of the shell. The tail with the branchial appendages is
always directed towards the orifice of the shell. The male is very
small and never leaves the female. The _Athelca cladophora_ is another
bopyrian living on the abdominal region of a pagurus, which always
chooses shells infested by Alcyonia. Another bopyrian, the _Prosthetes
cannelatus_, lives on the abdomen of an ordinary pagurus.
[Illustration: Fig. 29.--Phryxus Rathkei. A figure of the natural size
is given at the side.]
Mons. Bucholz has recently described a new kind of isopod, allied to
the lyriopes, which lives on the _Hemioniscus_. This isopod fixes
itself to a Balanus (_B. ovularis_), and the female preserves only
four of her segments with their appendages: she had fifteen, when
young. Thus she throws off nearly all her appendages which have become
useless. The male of this isopod, which inhabits the bay of
Christiansand, is not yet known. Another parasite of this group has
been observed by Fr. Müller at Desterro, on the coast of Brazil. It
bears the name of _Entoniscus porcellanæ_. The parasite which he 146
he discovered by the side of it on the same animal, and to which he
has given the name of _Lerneoniscus_, had perhaps introduced it. We
have seen examples of this kind among insects. Among the rich
materials which Professor Semper brought back from his voyage, there
was a Porcellana, which harbours on its exterior surface a very
remarkable isopod, whose recurrent development is no less decided than
that of the peltogasters. Dr. Kausmann has lately described these
curious organisms, to which he has given the name of _Zeuxo_. Another
isopod, with a no less decided recurrent development, has received
from the same naturalist the name of _Cahira Lerneodiscoïdes_.
We now come to an isopod which aims higher: he doubtless considers
that cray-fish and crabs walk too slowly for him; he therefore
addresses himself to a fish, the _Puntius maculatus_, which inhabits
the river Tykerang (Bandong) in Java. This isopod is called
_Ichthyoxenus Jellinghausii_. This isopod crustacean, living at first
in the same manner as the rest, looks out for a small cyprinoid fish,
thrusts itself like a trocar behind the abdominal fins, through the
scaly skin, and penetrates entirely into the abdominal cavity. The
male always accompanies its female. It is remarkable that she, in
contradistinction to many others, preserves all the attributes of her
sex. She does not change her form more than the other free crustaceans
of her order, and only differs from the male in size. It is well known
that in all these animals the male is always smaller than the female.
Mons. Jellinghaus, who first described this crustacean, observed that
all fishes which he caught had, without exception, the small ones 147
as well as those which were larger, a couple of these parasites in
their stomach. We allude to it here, but we might as well call this
_Ichthyoxenus_ a messmate as a parasite.
On the coast of Brittany, among the many _Labri_, which are
distinguished for their vivacity, and for the variety of their
colours, is found a small species (_Labrus Cornubiensis_), on which is
usually seen an isopod which is no less curious. It is constantly
clinging to the sides of this fish, not far from the head, at the
bottom of a hollow made under the scales. Naturalists have known this
acolyte by Mons. Hesse's works.
This _Leposphilus_ (for this is the name which has been given to it),
though it does not prefer the scales to any other organ, forms a
lodging for itself in the sides of this little Labrus, and takes up
its abode there with its family. We cannot assert that it has chosen
this refuge without any hope of returning, since both the sexes still
keep their organs of locomotion.
At the last congress of German naturalists at Wiesbaden, Dr. Kossmann,
who has had the opportunity of examining the rich materials brought
from the Philippine Isles by Professor Semper, gave an excellent
account of the result of his careful observations on some other
crustaceans still more remarkable, the _Peltogasters_ of which we have
spoken before. In the course of this, he described an isopod with a
development as completely recurrent as that of the peltogasters, whose
rank among cirrhipeds is perfectly established.
Most of the inferior crustaceans require assistance from others: some
might be correctly arranged as messmates, but the whole category of
the Lerneans is so low in development that Cuvier placed them by 148
the side of the helminths. These creatures possess as soon as they are
born, all the attributes of their class, and wear the dress of free
crustaceans; as they approach mature age, they choose a neighbour,
instal themselves as conveniently as possible in one of his organs,
and get rid of all their apparatus for fishing and hunting. The sexes
are usually separated, and as the female is specially devoted to the
cares of her progeny, she is the first to give up her liberty.
Sometimes the male, not content with leaving to her all the trouble of
providing for the family, demands from her his daily food, and
establishes himself like a spermatophore on her sexual organs. It is
only right to say that in this case, the male sex is far from being
the stronger, for he is often less than the tenth or even the
hundredth part of the size of the female. At last we see the female
lose her claws and her swimming apparatus, while the male keeps his
carapace with all his appendages of the senses and of locomotion. The
difference between the two sexes is so great in some species, that it
would be impossible to imagine that a brother and sister could assume
such dissimilar forms, unless we had watched them from the time when
they first issued from the egg. The female is a kind of puffed-out
worm, and the male resembles an atrophied acarus. This explains why
the female was known so long before the male, whose office is only
that of reproduction. Nordmann, during his residence at Odessa, was
the first to begin these researches, which have been continued by
Messrs. Metzger and Claus.
It is known that the Lerneans attach themselves to their hosts by
indissoluble bonds, only becoming parasites after they have passed 149
their youth in complete independence, and have all possessed the
graceful forms so characteristic of the _Nauplius_ and the _Zoë_. When
they first leave the egg, they swim about in freedom, but at length
some day the female, thinking of a family, looks out for a neighbour
that can give her the assistance she requires, fixes herself on his
skin, and rapidly develops till she is two or three hundred times as
large as the male; her head, her body, and her stomach become of a
monstrous size, a part of her head is often anchylosed in the bones of
her host; the lernean remains suspended as a sort of festoon, to which
are afterwards joined two ovisacs filled with eggs. Fig. 30 is a
lernean of a fresh-water fish, represented at different periods of its
existence.
[Illustration: Fig. 30.--Tracheliastes of the Cyprinæ. 1, larva, as it
leaves the egg; 2, larva, more advanced; 3, adult female, attaching
itself before and behind to two ovisacs (Nordmann).]
The lerneans are the most remarkable of all parasites with respect to 150
their physical degradation. They are met with on all aquatic animals,
commencing with the cetacea, and extending to the echinodermata and
polyps; but it is especially on fishes that they are most abundant.
They live on the skin or the gills, and sometimes establish themselves
in the nostrils and on the eye-ball. They often hang on the outside,
but we find some which hide themselves in the substance of the skin,
and have no communication with the exterior except by a narrow
orifice.
Some elegant lerneans, which resemble a living pen, are called
_Penellæ_; their head is divided into several branches, which plunge
like roots into the tissues and even into the bones, so that the head
and all the body remain suspended, as well as the ovisac tubes, to a
long and but slightly flexible neck. They live on the body and the eye
of certain fishes; some of great size are found in the Indian sea, but
the most remarkable are those which have been observed on the skin of
some of the cetacea.
The _Penella crassicornis_ lives on a hyperoodon; the _Penella
balænoptera_ on a _Balænoptera musculus_ among the Loffoden Isles; the
_Lerneoniscus nodicornis_ on a dolphin; the great shark of the coasts
of Ireland (_Scimnus glacialis_) generally has a lernean on its eye.
My son brought from Rio de Janeiro some Scomberidæ, whose skin is
covered with penellæ; and the charming fishes so abundant on the
Belgian coasts, which are called _Sprot_ by the fishermen of the
country, often have round their eyes strings which might be taken for
marine plants, and which are in reality only penellæ. We have found
sometimes many individuals on the same fish, stretching from the head 151
to the caudal region by means of their oviferous tubes, which in
certain seasons acquire a pale green tint.
The true Lerneans, such as the _Lernea branchialis_, a species that
was the earliest known upon the different Gadidæ, and which we have
observed on the _Callionyme lyra_, greatly resemble the Penellæ, but
their body and their head are much twisted, and with the coils of
tubes which contain the eggs, you might take them for a ball of
thread. (Fig. 31.)
[Illustration: Fig. 31.--Lernea branchialis, attached to the gills of
Morrhua luscus.]
The Sphyriones called _Leistera_ have also a most singular form, and a
new species has been recently observed on a fish from the Straits of
Magellan. The _Conchoderma gracile_ lives on the branchiæ of the _Maïa
squinado_, the sea-spider of the Adriatic, and Mons. W. Salensky of
Charkow, found a copepod crustacean, the _Sphæronella Leuckarti_, in
the egg-pouch of an _Amphitoë_. The latter parasite has very peculiar
characters of conformation and embryonic evolution.
Among the molluscs, the Tunicates give lodging to the greater number
of lerneans; in the cavity which is before the mouth, and by which the
food passes, some are found which can scarcely be recognized, and
which remain there to smell out a feast. The _Aplidium_ of the coasts
of Belgium gives lodging to some which are very curious, and which we
have named _Enterocola fulgens_, on account of their colours. The
_Notopterophorus_ establishes itself on the body of the _Phallusia
mamillaris_, and a certain number of these parasites are found on the
annelids. Professor Sars of Christiania, and Claparède have carefully 152
described them; and the latter saw on the _Spirographis Spallanzani_
of the bay of Naples, a female which he called _Sabelliphilus Sarsii_.
The genera _Selius_, _Silenium_, _Terebellicola_, _Chonephilus_,
_Sabellacheres_, _Nereicola_, &c. infest all the annelids; the
_Eurysilenium truncatum_ lives on the _Polinoë impar_, the
_Melinnacheres ergasiloïdes_ on the _Melinna cristata_.
The echinodermata and the polyps are not free from lerneans; thus the
_Asterochoeres Lilljeborgii_ fixes itself on the _Echinaster
sanguinolentus_, and we have found a very beautiful species in
Brittany on an Ophiurus; the _Loemippa rubra_, allied to the
_Chondracanthi_, lives upon the _Pennatula rubra_, the _Laura
Girardiæ_, according to Mons. Lacaze Duthiers, feeds on an Antipathes.
A Loemippus (_Proteus_) lodges in the cavity of the body of the
_Lobularia digitata_ of Delle Chiaie; and lastly, the _Enalcyonium
rubicundum_ is sheltered by the _Alcyonium digitatum_.
There are certain worms which are free when young, and only become
parasites at a later period of their evolution. We will give a few
examples.
The Medina, or Guinea worm (_Filaria Medinensis, dracunculus_)
(Fig. 32), is the terror of travellers who visit the coast of Guinea;
it is common, not only on the western coast of Africa, but also in
many other parts of this vast continent, and has been recently found
in Turkistan and South Carolina (Mitchell). It was formerly thought
that this Filaria could introduce itself directly through the skin as
a microscopic embryo; but Mons. Fedschenko, after some observations
made on the spot, and corroborated experimentally afterwards by
Leuckart, is of opinion that this worm is transmitted by means of the
Cyclops, a little fresh-water crustacean. Thus the parasite is
received by means of the water which is drunk; and this remark is 153
the more important since it will henceforth be only necessary to make
use of carefully filtered water in order to guard against it. At the
end of six weeks, the presence of the animal is revealed by tumours,
the true nature of which is not ascertained at first; then some wounds
appear, caused not directly by the worm, but indirectly in consequence
of the dissemination of its eggs. The Filaria at last is so entirely
atrophied that Professor Jacobson, after having seen it alive on one
of his patients at Copenhagen, wrote to Blainville: "This Medina worm
is not really a worm, it is a sheath full of eggs." In fact, all the
internal organs disappear and nothing exists there except the eggs and
their embryos.
[Illustration: Fig. 32.--Young Filaria of Medina; 1, Anterior
extremity; _c._ Mouth; 2, Caudal extremity; _d._ Anus; 3, Section of
the Body.]
The Filaria is not allied to the _Mermis_, as was formerly thought;
its organization is different, and its organs become atrophied in a
very different manner. The _Gordius ornatus_, brought from the
Philippines by Professor Semper, has given us an opportunity, by
different anatomical observations, to correct many errors, especially
with respect to the digestive apparatus (Grenacher). The _Filaria
immitis_ is a species found by Mons. Krabbe in a dog which died of 154
a disease to which these animals are subject; it lived in the heart,
and twelve individuals, ten females and two males, were found to be
lodged there. Mons. Bap. Molin has published a monograph on the
Filariæ, giving the characters of 152 species met with in molluscs,
fishes, amphibians, reptiles, birds, and mammals: it seems evident
that many species have been confounded under the same name.
A small worm, of the size of a slender pin, but much shorter, lives in
a manner somewhat analogous to that which we have before described. It
is known under the name of _Leptodera_. In order to find it, we have
only to search in the woods for the first snail that we meet with,
which is distinguished by its orange or black colour: if we prick with
a pin the fleshy foot of the mollusc, we shall see torrents of round
worms come out, wriggling like microscopic serpents. These worms also
leave their retreat, if we cause the foot to contract by touching it
with some acid, or if we place the snail in water. The Leptoderæ are
especially remarkable for two fringes which float by the side of their
tail, which characteristic suggested the name given to them by
Professor Schneider. These fringes so easily fall off, that the
greater part of those which have become free have none of these
appendages. When placed in fresh or decaying animal matter, in water
or in damp earth, these worms, agamous when in the foot of the
mollusc, rapidly become sexual and perfect. Thus the snail serves them
as a _crèche_, and the adult worm has no need of external help when it
has grown old.
Professor Pagenstecher found at Ostend, on the _Nicothoë_ of the
lobster, nematodes which he arranged among the Leptoderæ. This is 155
another instance of a parasite on a parasite.
While speaking of these worms, I will allude to a nematode which I
observed under very singular circumstances. I had a considerable
number of skeletons or, I should rather say, separate bones, exposed
to the sun upon a roof to whiten; among these skeletons there were
several hyperoodons and other cetacea. All these bones had remained
for a certain time in horse-dung in order to hasten the decomposition
of the soft parts. They had been in the open air for several weeks,
and were slowly bleaching; it had rained nearly every day. Towards the
end of the month of August, I examined some of the vertebræ, and found
them quite black on the upper part. Below, I discovered a mass of
syrupy matter, slightly yellow, like pus that has recently issued from
a wound. The sun was shining full upon the bones at this time; looking
at them more closely, I saw this pus issuing from the holes which
convey nourishment to the substance of the vertebræ; it seemed that
the inside of the bones was in full fermentation. Examining it with
some attention, I perceived that the whole surface was in motion; an
undulatory wriggling covered it as if a ciliated skin had been
stretched above the orifices. I took a little of this matter on the
point of a scalpel, and observed it with the microscope, and what was
my astonishment when I saw the whole mass in motion as if under the
influence of a magic wand. When I slightly compressed it afterwards
between two slips of glass, there remained nothing before my eyes but
nematode worms of very small size wriggling over each other: I found
males by the side of their females; in the bodies of the latter 156
were eggs ready to be laid, and millions of embryos of every age
rolling over and struggling among the full-grown worms. Is this a
species of worm new to science? Is it a worm which lives in freedom
here, and parasitically elsewhere? The first female which presents
itself allows us to answer this question. It is not a parasitical
worm, at least under this form, because each female contains only one
or two eggs. Parasites have so few chances of arriving at their
destination, that two young ones would not be sufficient. They must
have hundreds or thousands, and then the chances are against them.
This worm is evidently a _Rhabditis_, but is it that which lives
in the earth, or an allied species? Future observations will perhaps
enable us soon to reply to these questions. We do not think that these
creatures could have been brought with the bones from the Shetland
Isles; they came rather from the horse-dung, and they multiplied
beyond measure in the spongy tissue of the bones, where they found
good cheer and a convenient lodging. A worm very nearly allied to this
exists in abundance in the dung of the cow, to which our regretted
colleague, the Abbé E. Coemans, had directed my attention, at the time
when he was studying the _Pilobolus cristallinus_.
That which decided us to make mention of the nematode of the bones, is
the singular history of an ascaris of the frog, whose young ones
resemble their parents neither in size, form, or manner of life. There
is one generation which can provide for themselves, and is composed of
males and females; and another which requires assistance, and only
consists of females; unless, indeed, those of the male sex are hidden
among the eggs; we refer to the _Ascaris nigro-venosa_, the principal 157
characters of which have been made known by Professor Leuckart. This
Ascaris is a true parasite, which, when it arrives at its destination,
where it finds lodging and food, leaves the lungs to go and inhabit
another organ. There is nothing surprising that certain worms pass
from the intestines to the stomach, mount thence to the oesophagus,
and sometimes come out of the mouth; but here we have decided changes
of abode in the same animal; that which shows, besides, that it is not
a simple accident, is that the animal is of a different sex according
to the apartment which it occupies; here, it is hermaphrodite, there
it is male and female. The Linguatulæ, indeed, migrate from the
peritoneum of the rabbit to the nasal fossæ of the dog: but the
_Ascaris nigro-venosa_ first lives in the lungs of the frog, then goes
to inhabit the rectum of the batrachian, or damp earth. In the lungs
it is very small and viviparous, and produces young ones which become
stronger than their parents. The generation which live in the lungs
are hermaphrodite, the others are dioecious; that is to say, the males
and females have hermaphrodites for their parents. We have thus a
mother, a simple female or hermaphrodite, very small, which produces,
not eggs but young ones fully formed; and instead of living, like the
mother, in the lungs, and breathing there with greater or less
facility, they go and lodge in the rectum, and become, not like their
mother, viviparous and hermaphrodite, but oviparous and of separate
sexes. They produce in their turn a race of giants, and instead of
following the example of their father or their mother, they all go and
lodge in the lungs like their grandmother.
If the hermaphrodite _Ascaris nigro-venosa_ alternately produces 158
individuals of separate sexes, that is to say, if the monoecii
produce dioecii, and the dioecii again monoecii, one cannot help
comparing this phenomenon to digenetic generation. This is one of the
striking discoveries made at the laboratory of Giessen, under the
direction of Rud. Leuckart. Since then, Professor Schneider, the
successor of Leuckart at the University of Giessen, has also studied
these worms. Professor Leuckart wrote thus to me a few days after this
discovery: "The _Ascaris nigro-venosa_ presents this peculiar
phenomenon, that, under the parasitical form, it produces fertile eggs
without the presence of males. The embryos which proceed from the eggs
become sexual worms at the end of twenty-four hours after they have
left the body. This fact was first observed by M. Mecznikow, while he
was working in my laboratory, and taking part in my researches. The
experiment which produced this result was suggested and directed by
myself, in order to continue my work on the development of the
Nematodes."
We do not know if this is the place to speak of an animal which
excited great attention some years ago, and which was thought to prove
the transformation of animals into each other. It is a parasite which,
under the form of a gasteropod, lives under peculiar conditions. It is
known by the name of _Entoconcha_. Discovered by J. Müller in an
echinoderm of the genus Synapta, its complete development has been
vainly sought to be discovered since that time. It is evidently a
gasteropod mollusc, allied to the Natices, and lives in the interior
of the body of a Synapta, but we do not yet know all the phases of its 159
evolution. It was at first thought that we had before us an echinoderm
in the act of transformation. I wrote to J. Müller immediately after
the discovery which he hastened to announce to me, to state that in my
opinion, this was only a new instance of parasiticism; parasites are,
however, so rare in this class of animals, and their mode of life is
so exceptional, that one ought not to be surprised that this fact did
not receive at first its true interpretation.
Professor Semper found at the Philippine Islands, in the _Holothuria
edulis_, another species of Entoconcha which appears to attach itself
to the anal vent of this echinoderm. He gave it the name of
_Entoconcha Mulleri_. We have in it a new example of the relations
which certain parasites bear to their hosts, and which are the same in
both hemispheres.
The _Lichnophoræ_ are infusoria, allied to the _Vorticellæ_, whose
form they assume; these are "mimic species," or mocking forms, of the
Trichodinæ. One species, the _Lichnophora Auerbachii_ lives on the
_Planaria tuberculata_; the other, the _L. Cohnii_, on the branchial
membranes of the _Psyrmobranchus protensus_.
The associations in the inferior ranks of animals have functions which
are of the highest importance; some to maintain harmony and health in
all that possess life, others to sow the seeds of death throughout
whole regions. There are, in fact, associations in the ranks of the
infinitely small creatures, which sometimes have the effect of
purifying and rendering more healthful, sometimes of destroying. It is
among these beings, invisible to the naked eye, that we must seek for
the cause of some epidemic diseases. We have here an example of 160
what certain groups of animals are able to accomplish. The crustaceans
everywhere perform the office of vultures to clear the waters from
dead bodies, whether large or small, and they are in general
sufficiently numerous to perform this police duty effectually. We may
say that without their aid the waters along the coasts and at the
mouth of rivers would grow speedily corrupt and unfit to support life.
Thus it sometimes happens that when the number of these beings is
insufficient, or the putrescible matter is in excess, we see the fish,
the molluscs, and even the crustaceans, perish one after the other.
[Illustration: Fig. 33.--_Gregarina_ of _Nemertes Gesseriensis_.]
The last of the parasites of this category are known by the name of
Gregarinæ. It appears that Goede was the first to make observations
upon them. Léon Dufour gave them the name which they still bear. They
have a very simple organization, and are formed only of a cell which
contains a nucleus: they live in the intestines of many invertebrate
animals, especially in the articulata. Let us imagine a body, long,
more or less transparent, with a smooth surface very like a spindle,
which glides about in the intestines, in the midst of the liquid
matter which it contains, without our being able to ascertain the 161
mechanism by which it moves (Fig. 33.) While young they are encysted,
and bear the name of _Psorospermiæ_. Fig. 34 represents one of
these sacs of Psorospermiæ from a cephalopod.
[Illustration: Fig. 34.--Sac with Psorospermiæ from the _Sepia
officinalis_.]
The gregarinæ live in their perfect form chiefly in insects,
crustaceans, and worms. Fig. 35 represents a gregarina very common in
the libellulæ. The largest species inhabits the intestines of the
lobster. My son has studied them very carefully, and published the
results in the bulletins of the Academy of Belgium.
[Illustration: Fig. 35.--_Stylorynchus oligacanthus_ from the larva of
the Agrion.]
Schneider has described a parasite which ought, no doubt, to be placed
among the gregarinæ; it lives in the testicle, as well as in the
salivary cells, of a planaria, the _Mesostomum Ehrenbergii_;
Schneider represents the various phases of its development. In the
autumn of 1871, nearly all the mesostomes perished through the
presence of these parasitical organisms: in the following year they
were rare.
Some years ago, Kölliker discovered on the spongy bodies of molluscs,
certain parasites, the nature of which appears still as enigmatical as
on the first day of their discovery. The Würzburg professor gave them
the name of _Dicyema_. We have had for a long time in our
portfolio some observations upon them, and at the close of the chapter
"On Parasites that undergo Transformations," we give a representation
of a Dicyema which we found in abundance on the _Sepia officinalis_
off the coast of Belgium.