CHAPTER VII. 138

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.

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.

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.

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.)

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.

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.

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.

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.

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.