Popular Science Monthly/Volume 8/April 1876/Animal Parasites and Messmates
| ANIMAL PARASITES AND MESSMATES. |
THE fight for a foothold in the animal world brings the combatants into many strange relations, few of which are more curious and interesting than those existing between the creatures popularly known as parasites and the animals which furnish them support. In these relations all grades of pauperism and criminality are represented. There is the miserable wretch that lives entirely at the expense of others, finding it easier to die than to help himself; the poor weakling, willing enough to do what he can, but sure to starve to death if left wholly unassisted; the petty thief that sneaks into his neighbor's premises and steals a portion of his store; and the audacious robber that boldly appropriates another's substance, and not unfrequently adds murder to his list of crimes. In his entertaining and instructive work on "Animal Parasites and Messmates,"[1] from which this article and its illustrations are mainly taken. Van Beneden makes these different degrees of dependence the basis of a rough but convenient classification, by which he separates, what have hitherto been known as parasites, into three groups, named respectively messmates, mutualists, and parasites.
The messmate is one that takes his place at his neighbor's table to partake with him of the product of the day's toil. He does not live directly at the expense of his host, but, abiding with him, obtains thereby better opportunities for securing a supply of food. This mode of getting a living is very common, and a curious thing about it is that animals comparatively high in the scale of organization do not scruple to quarter themselves upon others of much inferior grade. The fish known to naturalists as fireasfer lives in this relation. He takes up his lodgings in the digestive tube of a holothurian, and, regardless of the rules of hospitality, appropriates a portion of all the food that enters. He thus manages to get himself served by another better provided than he is with the means of fishing. Dr. Greef found at Madeira a holothurian over a foot long, in which one of these fishes was enjoying a peaceful and vigorous existence. Other fishes besides the fireasfer have been found in similar quarters; indeed, the situation appears a very favorable one for this mode of life, since not only fishes but crustaceans here take up their abode, sometimes in considerable numbers. Prof. Semper has seen holothuriæ in the Phillipine Islands which bore considerable resemblance in this respect to a hotel with its table-d'hôte.
A somewhat more excusable piece of pauperism is found in the case of an eel, which ensconces itself in the branchial sac of that curious fish known as the angler, or fishing-frog (Fig. 1), where he afterward plays the part of a messmate. Although the eels generally
Fig. 1.—The Angler-Fish.
get their living easily, the angler possesses fishing-implements which are wanting in them, and, when immersed in the ooze, it carries on a fishery sufficiently abundant for both. This relationship was first observed by Risso in the Mediterranean; the same fish in more northern seas has since been found to harbor, in like manner, an amphipod crustacean.
Another remarkable example of this kind of association among fish was made known by Reinhardt, of Copenhagen. A siluroid fish occurring in Brazil, and possessed of numerous barbules that make it successful as a fisherman, lodges in the cavity of its mouth some very small fishes, that for a long time were supposed to be young siluroids; it was believed that the mother brought her progeny to maturity in the mouth, as marsupials do in the abdominal pouch, or as some other fishes do. But this is a mistake. The supposed young are perfectly developed adult fish, that, instead of living by their own labor, prefer to install themselves in the mouth of a neighbor, and take tithes of the morsels which he swallows.
The little crab that makes its abode within the shell of the edible oyster (Fig. 2) is a true messmate, and the oyster is but one of many bivalve mollusks that give shelter and partial support to these diminutive crustaceans. These crabs, called by naturalists Pinnotheres, though in one sense dependents, are at the same time of great service to the animals within whose shells they receive protection. Van Beneden says of them: "The pinnothere is a brigand who causes himself to be followed by the cavern which he inhabits, and which opens only at a well-known watchword. The association redounds to
Fig. 2.—Oyster Crab.[2]
the advantage of both; the remains of food which the pinnothere abandons are seized upon by the mollusk. It is the rich man who installs himself in the dwelling of the poor, and enables him to participate in all the advantages of his position. The pinnotheres are, in our opinion, true messmates. They take their food in the same waters as their fellow-lodgers, and the crumbs of the rapacious crabs are doubtless not lost in the mouth of the peaceful mussel.... Little as they are, these crabs are well furnished with tackle and advantageously placed to carry on their fishery in every season; concealed in the bottom of their living dwelling-place, they choose admirably the moment to rush out to the attack, and always fall on their enemy unawares. Some pinnotheres live in all seas, and inhabit a great number of bivalve mollusks."
In the examples thus far cited, and in many more that have been observed, the dependent forms are free to depart whenever they choose, and are therefore called free messmates. Though for a time giving up their liberty, they sooner or later resume it, in possession of all their organs for fishing and locomotion, and in all respects fitted to live an independent life. There are others, however, that enter into the same sort of association, and make the relation a permanent one: these are known as fixed messmates. They are free in their youth, but, as maturity approaches, and the cares of a family are thought of, a host is selected in which they establish themselves, and, throwing aside their fishing and locomotive apparatus, they renounce the world, and even part with the most precious organs of animal life, not excepting those of the senses.
The most interesting fixed messmates are those cirripeds or barnacles which, under the names of Coronula and Tubicinella (Figs. 3 and 4), cover the skins of whales. They are, like all the rest, free while young, but later they take shelter on the back or on the head of one of these huge cetaceans, and, having once chosen their abode, are after-ward permanent tenants. Each whale lodges a particular species, and
Fig. 3.—Coronet Barnacle (Coronula diadema).
the manatee, marine turtles, and various sea-snakes, have also their different sorts. Others establish themselves on their own immediate relations and on other crustaceans. A pretty genus found near Cape Verd, living on the carapace of a large lobster, spreads itself over the
Fig. 4.—Burrowing Barnacle (Tubicinella trachealis).
centre of the lobster's back, and looks not unlike a bouquet of flowers. Fig. 5 shows a fixed messmate attached to a sertularian.
Mutualists, as the name suggests, are animals which live on each other; and, though usually confounded with messmates and parasites, they differ from both in making some sort of return for benefits obtained. Many insects shelter themselves in the fur of the mammalia or in the down of birds, and remove from the hair or the feathers the pellicle and epidermal débris which encumber them. At the same time they minister to the outward appearance of their host, and are of great use to him in a hygienic point of view. Animals living in the water are similarly served by minute crustaceans. These sometimes establish themselves on fishes, and, if there are no scales of the epidermis which annoy them, there are mucosities which are incessantly renewed in order to protect the skin from the continual action of the water. Among the insects found on the skins of mammals and birds that yield some return for the hospitality they receive, those belonging to the family Riciniœ, and commonly known as ticks, are very numerous. Among the many generic divisions, one of the most interesting has received the name of Trichodectes, it contains twenty species, one of which lives on the dog, another on the cat, another on the ox; in a word, there is a distinct species on each of the domestic mammals. The species infesting the dog has lately attracted especial attention, from the circumstance that it lodges the larva of the Tœnia cucumerina, a tapeworm common to dogs. The cock, the turkey, and the peacock, carry each a distinct species of Riciniœ, and oftentimes several species are found on a single bird. Fig. 6 represents a form which infests the pygarg or sea-eagle.
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| Fig. 5.—Ophiodendrum Abietinum on Sertularia abietina. |
Fig. 6.—Ricinus of the Pygarg. | |
Fishes harbor crustaceans instead of insects, frequently in enormous numbers. They live on the produce of cutaneous secretions, and thus, like the ticks, are of service to their hosts. The Caligi and Arguli, known usually as fish-lice, are among the most common of these, and both are elegant forms, that change but little in appearance in the course of their lives, and, although permanent tenants when once established, they retain their fishing-tackle and locomotive apparatus. The greater number of osseous fishes lodge Caligi on the surface of their skin, where the tiny creatures fix themselves by means of strong cables. Fig. 7 represents a species that lodges on the cod, and it in its turn affords a resting-place for another form—the Udonellœ.
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| Of the Natural Size. | Caligulus Elegans. Female. | |
| Fig. 7. | ||
A curious creature, with an equally curious function, that entitles it to a place among mutualists, was discovered some years ago among the eggs of the lobster, by Van Beneden, who thus describes it: "It is known that lobsters, as well as crabs, and the greater part of the Crustacea, carry their eggs under the abdomen, and that these eggs remain suspended there until the embryos are hatched. In the midst of them lives an animal of extreme agility, which is, perhaps, the most extraordinary being that has been subjected to the eyes of the zoölogist. It may be said, without exaggeration, that it is a biped, or even quadruped, worm. Let us imagine a clown from the circus, with his limbs as far dislocated as possible, we might even say entirely deprived of bones, displaying tricks of strength and activity, on a heap of monster cannon-balls which he struggles to surmount; placing one foot, formed like an air-bladder, on one ball, the other foot on another, alternately balancing and extending his body, folding his limbs on each other, or bending his body upward like a caterpillar of the Geometridœ, and we shall then have but an imperfect idea of all the attitudes which it assumes, and which it varies incessantly. It is neither a parasite nor a messmate; it does not live at the expense of the lobster, but on one of the productions of these crustaceans, much in the same manner as do the Caligi and the Arguli. The lobster gives him a berth, and the passenger feeds himself at the expense of the cargo; that is to say, he eats the eggs and the embryos which die, and the decomposition of which might be fatal to his host and his progeny. These Histriobdellœ have the same duty to perform as vultures and jackals, which clear the plains of carcasses. That which causes us to suppose that such is their appropriate office is, that they have an apparatus for the purpose of sucking eggs, and that we have not found in their digestive canal any remains which resemble any true organism."
True parasites are beings entirely dependent on their neighbors for support; unable to provide for themselves, they are fed wholly at the expense of others. It is generally believed that they are an exceptional class of organisms, constituting a group by themselves, and knowing nothing of the world outside the organ which shelters them. This is an error. Representatives of all the principal divisions of the animal kingdom below the vertebrate are found pursuing this mode of life. There are few parasites that are not wanderers at some period of their lives; and it is not uncommon to find some which live alternately as noblemen and as beggars. Many are paupers only during infancy, or at the approach of adult age, living at other times a comparatively free and independent life. Nor are all the members of a species necessarily parasitic; sometimes it is only the female that takes the relation of a dependent, the male continuing his nomad life. Again, there are cases where, the female being provided for, the male relies on her for support, and thus the charitable animal which comes to her help is laid under contribution by the whole family.
Parasites present an extraordinary variety of forms, and differ very widely in size and appearance, these differences being often remarkable between the sexes of the same species. The male of the urubu of Brazil has the usual form of a round long worm, while the female resembles more than any thing else a ball of cotton, not having the slightest analogy with the other worms of the order. As to the enormous proportions parasites may attain, Boerhaave mentions a bothriocephalus 300 ells[3] in length; and, at the Academy of Copenhagen, it was reported that a solitary tapeworm (Tœnia solium) had been found 800 ells long. Parasites are found in every region of the globe, but, like other animals, they observe the laws of geographical distribution. Some, like the leeches, take their food, and then detach themselves until the demand for food returns, never becoming identified for any length of time with their host. Others, like the lernæans, commence their parasitic existence when approaching maturity, and thereafter are permanent dependents; others, again, like the ichneumons, begin life as parasites, and on reaching maturity assume and maintain an independent existence; while still others, like the tænia, are parasitic from first to last, although changing their abode at a certain stage of development.
All animals, man included, have their parasites, which usually come from without, those entering the body being generally introduced with the food or drink. No organ is exempt from their incursions, as they have been found in the brain, the ear, the eye, the heart, the blood, the lungs, the spinal cord, the nerves, the muscles, and even the bones. Cysticerci have been seen in nearly all these situations, and worms of various kinds are common in the cavities of the body, as well as in many of the solid organs, such as the muscles, liver, and kidneys. As a rule, those which inhabit a temporary host install themselves in a closed organ; in the muscles, the heart, or the lobes of the brain; those, on the contrary, which have arrived at their destination, and which, unlike the preceding, have a family, occupy the stomach, with its dependencies the digestive passages, the lungs, the nasal fossæ, the kidneys, in a word, all the organs which are in direct communication with the exterior, in order to leave a place of issue for their progeny.
A single animal may carry, not only a great number of individuals of the same species, but many different species of parasites, and this, too, without any apparent impairment of health. Indeed, in some countries their presence is considered indispensable to the highest health, the Abyssinians, for example, deeming themselves below par unless they nourish one or many tapeworms. Nathusius speaks of a black stork which lodged 24 Filariœ in its lungs, 16 Syngami tracheales in its tracheal artery, more than 100 Spiropterœ within the membranes of the stomach, several hundred of the Holostomum excavatum in the smaller intestines, 100 of the Distoma ferox in the large intestines, 22 of the Distoma hians in the œsophagus, and a Distoma echinatum in the small intestine. In spite of this affluence of lodgers, the bird did not appear to be the least inconvenienced. Krause, of Belgrade, mentions a colt, two years old, which contained more than 500 Ascarides, 190 Oxyures, 214 Strongyli armati, several million Strongyli tetracanthi, 69 Tœnia, 287 Filariœ, and 6 Cysticerci. Well supplied as these animals appear to have been, when we consider the number of eggs a single worm may produce, the wonder is that parasites are not more numerous than they are: 60,000,000 eggs have been counted in a single nematode, and in a single tapeworm more than 1,000,000,000 eggs have been found!
While nearly all animals, including parasites themselves, are made to contribute to the support of others, those to which man gives food and lodging are of greatest interest, and he is by no means scantily provided with this class of dependents. Four different cestodes, or tapeworms, live in his intestines; three or four Distoma lodge in his liver, intestines, or blood; nine or ten hematodes, or round worms, inhabit his digestive passages or flesh; and cysticerci, echinococci, and hydatids, are also among his guests. He provides a living for three or four kinds of lice, for a bug, for a flea, and two ascarides, without mentioning certain inferior organisms which lurk in the tartar of the teeth, or in the secretions of the raucous membrane of the mouth. Some of these are confined to him exclusively, others may also find a home on the lower mammalia; some make his body their home while passing through a single stage of development, beginning or finishing the process, as the case may be, in the body of another animal; and others, again, are but day-boarders, taking their meals at his expense, and finding lodgings elsewhere.
Leeches are true parasites, although asking only food and taking care of themselves in the intervals of their meals. They suck the blood of their victim, and, when gorged to the very lips, fall off and perhaps for many weeks have no further need of assistance. The vampires of South America obtain support in a similar way, and are just as truly parasitic, although otherwise leading an independent life. The best-known leeches are those which prey on man and other mammals; but some are found which attack animals of still lower grade, especially the fishes. The organization of the leech appears always to be proportioned to that of the host which it frequents, the lower the grade of the latter the simpler the structure of the former. Those living on the mollusks are inferior to those found on fishes, and these again rank below the sorts that attack the mammalia. Fig. 8 (1, 2, 3, 4) shows the different appearances assumed by the skin after a leech-bite; Fig. 9 represents the structure of the jaws; and Fig. 10 is a longitudinal section of the body of the leech. The letters Fig. 8.
| Fig. 8.—Different Forms of the Bite of a Leech. | |
| Fig. 9.—1. Sucker, open; a, Jaws. 2. One of the Jaws magnified. | |
| Fig. 10.—Section of a Leech: a, Anterior Sucker; b, Posterior sucker; c, Anus; d, Stomach; œ, Œsophagus; i, Intestine; s s, Glands of the Skin. |
Fig. 11.—Gnat (Culex pipiens), Larva and Nymph.
Gnats or mosquitoes are parasites that get their living in much the same way as the leeches, that is, they suck the blood of other animals, man being their most common victim. They differ from the leeches, however, in the fact that only the females are greedy of blood, the males living on the juices of plants. The females pierce the skin by means of an auger with teeth at the end, and after sucking their fill distill into the wound a liquid venom which occasions the irritation that follows the bite. Fig, 11 shows the form of the larva and nymph of this insect. The former will be recognized as the little "wriggler" that may be seen in such numbers in stagnant water in summer. Fortunately, these insects are harmless until they acquire wings, and after that their life is a short one; but, unfortunately, they breed at an enormous rate, and thus maintain the supply, to the infinite annoyance of man and other tender-skinned animals.
Another blood-sucking parasite of both man and beast, whose staying tendencies are proverbial, is the louse. Fig. 12 represents the species that inhabits the head of man. The mouth of this insect consists of a sucker contained in a sheath, without articulations. It is armed at the point with retractile hooks, within which are four bristles
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| Fig. 12.—Louse of the Head. | Fig. 13.—Louse of the Head. 2, 3, Sucker. | Fig. 14.—Louse of the Head, Claw. | ||
that aid in breaking through the skin. They have climbing feet terminated by pincers, with which they maintain their hold on the hairs. The sucker and claw are illustrated in Figs. 13 and 14. The nits, or eggs, hatch in five or six days after they are laid, and in eighteen days more the creature is able to reproduce its kind. Leeuwenhoek calculated that two females might become the grandmothers of 10,000 lice in eight weeks.
A not less annoying parasite that lives on the blood of man and the higher animals is the flea. Both male and female get their living in this way, and even the larvæ are supplied from the same sources by the mother, who sucks for herself first, and then divides with her young ones. The ordinary flea (Pulex irritans, Fig. 15) is common in both Europe and North America. It may be called a fly without wings, and, together with others of its kind, forms a distinct family under the name Pulicidœ. The four principal species are Pulex irritans of man, Pulex canis of the dog, Pulex musculus of the mouse, and Pulex vespertilionis of the bat. Great numbers of human fleas, half as large as the common fly, are found in summer on the sandy shores of the Mediterranean, in the neighborhood of Cette and Montpellier. Their presence in this locality is due solely to the circumstance that large numbers of persons of both sexes and all classes come to these places to bathe, and, laying their clothes upon the sand, leave there a part of their vermin. Van Beneden suggests the surgical employment of the fleas an homeopathic phlebotomist, and recommends this region as an excellent source of supply in case his suggestion is adopted. The largest fleas are found upon the bat; they sometimes annoy the horse, and there is a species peculiar to monkeys.
Fig. 15.—Human Flea (Pulex irritans).
The minute creatures known as Acari, or mites, are most of them parasitic, and they are very widely distributed. They are not true insects, but belong to the Arachnida, having four pairs of legs like the spiders, with head and thorax closely united. The group includes those disgusting creatures the itch-mites, magnified representations of which are shown in Figs. 16 and 17. The mammalia have each
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| Fig. 16.—Sarcoptes Scabiei, Or Male Acarius of the Itch. The Lower Surface. | Fig. 17.—Sarcoptes Scabiei, Female. The Upper Surface. | |
The true parasites just described, and many others. like them, are nourished by the blood of their neighbors, but they never establish themselves in the organs of their host, being free throughout their lives. There is another class that live in freedom while young, but when arrived at mature age, and the cares of a family are soon to be assumed, they change in appearance, choose a host, and settle down for life. The chigoe, a parasite of man in South America, is one of these. It is only the female, however, that demands both lodging and provisions, the male (Fig. 18) being contented with pillaging his victim as he passes by. It is a small species, which pierces the shoes and clothes with its pointed beak (Fig. 19), and penetrates into the substance of the skin, generally selecting that of the toes. The male, as
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| Fig. 18.—Male Chigoe. | Fig. 19.—Head of Chigoe. |
just remarked, takes his food and resumes his wanderings, but the female seeks a hiding-place for permanent abode, and then grows to such a monstrous size that the entire insect appears to be nothing more than a mere appendage to the abdomen, as may be seen in Fig. 20, Besides man, this parasite infests the dog, the cat, the pig, the goat, the horse, and the mule.
Another form coming within this category, and the terror of travelers on the coast of Guinea, is the Guinea-worm, Filarla medinensis (Fig. 21), also found in other parts of Africa, and said by Mitchell to have been observed in South Carolina. It was long supposed that this filaria could introduce itself into the cellular tissue of the body directly through the skin, in the form of a microscopic embryo, but several recent observers concur in the belief that it is transmitted by means of the cyclops, a little fresh-water crustacean. This is swallowed in drinking-water, and at the end of six weeks the presence of the filaria is revealed by tumors, which later develop into open sores, caused not by the worm itself, but by the dissemination of its eggs. The filaria at last is so entirely atrophied that Prof. Jacobson, after seeing 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 is found in their place except the eggs and their embryos.
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| Fig. 20.—Female Chigoe. | Fig. 21.—Young Filaria of Medina. 1. Anterior Extremity; c, Mouth. 2. Caudal Extremity; d. Anus. 3. Section of the Body. | |
The ichneumons and many other insects that lay their eggs in the living larvae of other species, belong to a class of parasites that begin life as dependents, but that become free and self-supporting on arrival at adult age. The Œstrus or gadfly of the horse (Fig. 22), is
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| Hinder Part. | Fig. 22.—Œstrus of the Horse. | Anterior Part. |
thus dependent in its early life. But, instead of making their attacks on those of their own class, the gadflies prefer to install themselves on mammals, and sometimes even on man. The eggs are received into some cavity of the body, nostrils, stomach, or a hole in the skin, where they hatch and where the larvæ feed until the adult state is reached, when they escape and afterward live in freedom.
There is a large class of parasites generally known as worms, characterized by the circumstance that during their lives they undergo certain strange transformations that can only take place by the passage of the creature from one animal or host to another. The eggs are swallowed by some animal, usually a vegetable feeder, they hatch within its body, enter its tissues, and remain in a state of incomplete development until transferred to the stomach of another animal which has eaten the flesh of the first one. Here development recommences, and goes on to completion, when the process of reproduction begins. Each species of worm has its particular animals, through the agency of which these changes occur, and, if in its passage it gets off the proper track, that is, enters the wrong animal, it must either perish, or, as sometimes happens, find its way by a second transfer into the body of its destined host. The tapeworm of man. Tœnia solium (Figs. 23 and 24), is a member of this group, belonging to the Cestoidea, or ribbon-like worms. These cestoids are found in all classes of vertebrate animals. They exist in two principal forms. The first or vesicular form resembles somewhat in appearance the finger of a glove partly drawn inward. In this shape they are always lodged in the midst of the flesh, or in a closed organ, surrounded by a cyst, and
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| Fig. 23.—Tænia Solium, or Solitary Worm. | Fig. 24.—a, Rostellum; b, Crown of Hooks; c c, Suckers. 1. Scolex of the Tænia solium. 2. Hooks expanded; a, Heel of the Hook. |
| a, head, or scolex; b, tape formed of many individuals, the last of which, completely sexual, separate under the name of Proglottides, and represent the adult and complete animal. Each solitary worm is a colony. | |
thus circumstanced the worm is harbored by a host which is to serve as a vehicle to introduce it into its final host. It is a parasite on a journey, and usually bears the name of Cysticercus (Fig. 25). In the second shape it is like a ribbon, it attains a great length, always occupies the intestine, and is mainly occupied in producing eggs, which it turns out by the million. A description of Tœnia solium, the most common tapeworm of man, will enable us to understand all the others. Under its first, or vesicular form, this parasite comes from the flesh of the pig, where it is often found in large numbers, when the pig is said to be "measly." This condition of the pig has been attributed to damp, to feeding on acorns, to hereditary causes, to contagion, and various other influences, but none of these notions are correct. The
Fig. 25.—Cysticercus.
a, Upper Part of the Vesicle; b, Place where the Vesicle is about to separate: c, Neck of the Worm; d, The Head, showing the Suckers and the Crown of Hooks.
only true cause is the introduction of the eggs of Tœnia solium into the intestines of the pig These eggs, or fragments of tænia containing them, are swallowed by the animal. In the gastric juice of its stomach the eggs are set at liberty, lose their shells, and there issues an embryo singularly armed. It carries in front two stylets, in the axis of the body, and on the right and left sides two other stylets, which act like fins. These embryos bore into the tissues as the mole burrows in the soil. The middle stylets are pushed forward like the snout of the insectivore, and the two lateral stylets act like the limbs, taking hold of the tissues and forcing the head forward. In this manner the embryos perforate the walls of the digestive tube, and find their way, by means of the blood or otherwise, to the organ or tissue which is to become their temporary home. When arrived at this point they surround themselves with a sheath; their stylets, no longer of use, decay; and at one of the extremities appears a crown of new hooks, quite different from the former ones, which will serve to anchor their progeny in the new host to which they are ultimately destined. This vesicular worm, or cysticercus, fully formed, and without undergoing any change, waits till its host, the pig, or that part of him which it inhabits, is eaten, and, if its life has not been destroyed on its way through the frying-pan, it wakes up in some human stomach. Once there, it instantly quits its torpid state, gets rid of its useless envelopes, passes into the intestine, and, by means of its hooks and suckers, attaches itself to the intestinal walls, when it begins to grow with great rapidity, a length of many feet being attained in a few weeks. The part attached is the mother or head of the tænia, and until this is dislodged the worm goes on producing segments, or more properly proglottides, each of which is a perfect sexual being loaded with eggs. These are successively detached and escape with the evacuations, to be swallowed, perhaps, by some other pig, in whose flesh a new crop of cysticerci will soon develop. An egg of the Tœnia solium may be swallowed by a man instead of passing into the stomach of a pig. It is hatched in his stomach precisely in the same manner, and the embryo takes up Its lodging in some inclosed cavity. Some have been found in the eyeball, in the lobes of the brain, in the heart, and in the muscles. Whatever symptoms its presence may give rise to, it obviously has no chance for further progress, having selected the wrong vehicle to travel in. Man harbors not only the Tœnia solium, but another species very similar which naturalists have only learned to distinguish from it during the last few years, the Tœnia medio-canellata. Its cysticercus is found in beef, and is introduced when the meat is eaten in a raw or partially-cooked state. Tœnia nana and Tœnia lata are the names of other tapeworms inhabiting man, but both are limited in geographical distribution. The former is found only in Egypt, and the latter is confined to Russia, Poland, and Switzerland.
All these internal parasites, including the Trichina spiralis, which we have not space to speak of further, are introduced into the body either with the food or the drink, and a simple and effectual means of avoiding them is, to thoroughly cook the food and carefully purify the water.
