Nematocyst

“ If a small animal comes in contact with the tentacles of the polyp, it instantly becomes paralysed. If it adheres to the tentacle, it perishes; but if, as is often the case, it does not do so, it soon recovers the power of movement. Animals which do not adhere are generally those (such as ostracod crustacea) which have a hard integument without weak spots. Nematocysts of both kinds shoot out their threads against prey with considerable violence, the discharge being effected, apparently in response to a chemical stimulus, by the sudden uncoiling of the thread and its eversion from the capsule. ”

“ In the majority of Coelenterates the nematocysts are confined to the ectoderm, but in many Anthozoa, Scyphozoa, and Siphonophora they are found in tissues that are certainly or probably endodermic in origin. It has not been definitely proved in any case that the cnidoblast cells that form these nematocysts have originally been formed in the endoderm, and it is possible that they are always derived from ectoderm cells which migrate into the endoderm. ”

“ The external epithelium is flat on the ex-umbral surface, more columnar on the sub-umbral surface, where it forms the muscular tissue of the sub-umbrella and the velum. The nematocysts of the ectoderm may be grouped to form batteries on the tentacles, umbrellar margin and oral lappets. In places the nematocysts may be crowded so thickly as to form a tough, supporting, “chondral” tissue, resembling cartilage, chiefly developed at the margin of the umbrella and forming streaks or bars supporting the tentacles (“Tentakelspangen,” peronia) or the tentaculocysts (“Gehörspangen,” otoporpae) . ”

“ Under these conditions the cell can take in food-particles as bulky as the eggs of Rotifers and Copepods, by the protrusion of a pseudopod at the junction of the two grooves. As in most coloured forms an eye-spot is often present, a cup-shaped aggregation of pigment, with a lenticular refractive body in its hollow. A contractile vacuole, here termed a "pusule," occurs in many species, communicating with the longitudinal groove by a canal. Nematocysts (see p. 246 f.) are present in Polykrikos, trichocysts (see p. 142) in several genera. ”

“ A third point of dispute is whether the nematocysts are formed in situ, or whether the cnidoblasts migrate with them to the region where they are most needed; the fact that in Hydra, for example, there are no interstitial cells in the tentacles, where nematocysts are very abundant, is certainly in favour of the view that the cnidoblasts migrate on to the tentacles from the body, and that like the genital cells the cnidoblasts are wandering cells. ”

“ Among animals which possess this histological structure the Cœlenterata stand alone in having an alimentary canal, which is open at its inner end and communicates freely by this aperture with the general cavity of the body," and "all (unless the Ctenophora should prove a partial exception to the rule) are provided with very remarkable organs of offence or defence, [144] called thread-cells or nematocysts." ”

“ Some are to be met with in all the organs—the brain, the eye, the muscles, the heart, the lungs, the tracheal artery, the frontal sinus, the digestive tube, the skin, and even in the blood. Sometimes the two sexes live under the same conditions; sometimes the male is dependent on its female, or else one generation is parasitical, and the next is independent. There is a great diversity with respect to development. Some nematodes, like trichinæ, are developed so rapidly, that the embryos are already perfect in the egg before it has quitted its mother. ”

“ Minute Nematodes abound in moist soil, around the roots of plants, etc., and may easily be detected with the aid of a lens wriggling about amongst the particles of sand and earth. Of the animal parasites perhaps the most familiar is the "round worm" (Ascaris lumbricoides, Figs. 66 and 67) , which inhabits the alimentary canal of man; others are common in domesticated animals, as A. mystax in the cat and dog, and A. megalocephala in the horse and ox. They are also found living parasitically in plants (Fig. 77) , causing the formation of galls and other pathological growths ”

“ The Metazoa supply examples of intestinal parasites from the classes Annuloida and Nematoidea. To the former class belong the various tapeworms found in the small intestine of man. They, like other intestinal parasites, are destitute of any power of active digestion, simply absorbing the nutritious proceeds of the digestive processes of their hosts. Nematode worms infest both the small and large intestine; Ascaris lumbricoides, the common round worm, and the male Oxyuris vermicularis are found in the small bowel, the adult female Oxyuris vermicularis and the Tricocephalus dispar in the large. ”

“ There is scarcely a stage which may not be parasitic; the eggs, the larvae, the adults are all in some cases free, in others parasitic, and in many cases first the one and then the other.
2. Occurrence and Effect of the Parasite on the Host.—Von Linstow states that the only law that can be derived inductively from the study of the life-history of Nematodes is that those that live in animals never pass through all their stages of development in the same organ; consequently, in considering the distribution of {163}the parasites within the body of their host we have a double habitat to consider. ”

“ Of the various classes of animals which are more or less parasitic, the Nematodes show less difference between the free-living and parasitic members of the group than obtains in any other class. With few exceptions, such as Sphaerularia, Allantonema, and one or two others, the parasitic forms have undergone but little degeneration. It is true that they have no eyes such as the free forms often possess, but in other respects, such as in the nervous, muscular, and digestive systems, they do not show any marked retrogression ”

“ The parasite may be found in its adult state within the lumen of the cæcum or colon, but it is found more frequently in the larval stage lodged beneath the mucous membrane in cysts varying in size from that of a pin's head to that of a hazel-nut, each cyst containing a coiled-up worm. The species peculiar to apes and monkeys are likewise found during their larval stage within small hæmorrhagic cysts beneath the mucosa of the cæcum and large intestine. The parasites are blood-suckers; their intestines are crammed with erythrocytes. ”

“ Not only unicellular organisms, such as bacteria and protozoa, but also the eggs, embryos and larvæ of parasitic worms have been found to be transported by house-flies. Ransom (1911) has found that Habronema muscæ, a nematode worm often found in adult flies, is the immature stage of a parasite occurring in the stomach of the horse. The eggs or embryos passing out with the feces of the horse, are taken up by fly larvæ and carried over to the imago stage. ”

“ Another common nematode, the Oxyurus vermicularis (Fig. 69) , a parasite of man, is a small worm of the size of a fine pin, which often multiplies in the rectum of children, causing intolerable itching. It is by means of their microscopic eggs that they penetrate into the system; these are hatched in the stomach, and are completely developed at the end of eight or ten days. They pass from the anus in great numbers.
Fig. 68.—Trichocephalus of man.—1, female, a, cephalic extremity, b, caudal extremity and anus, c, d, digestive tube and ovary, e, orifice of sexual apparatus. ”

“ Nevertheless, in no case does true encystment occur, the sex-cells being developed practically free. The female gametes are large and egg-like; the males, minute and sickle-like, but with no flagellum and apparently non-motile. While many of the zygotes (“amphionts”) resulting from copulation pass out to the exterior, to infect a new host, others, possessing a more delicate investing-membrane, penetrate in between the intestinal cells, producing a further infection (auto-infection) . Numerous sporozoites are formed in each zygote. ”

“ It is a small worm, seldom reaching more than two or three inches in length—Rudolphi records it as reaching 80 mm.; the head is provided with four suckers, and, like the other two species, is devoid of hooks; the proglottides are thick and short, and the colour creamy white.
Nematodes we find in all parts of the body. This group of worms, which contains so many parasites, is of much importance to those interested in Horses and Horse-breeding in all parts of the world, for they are often the cause of serious epizootic attacks, occasioning very considerable loss. ”

“ These free-living forms are found everywhere—in salt and fresh water, in damp earth and moss, and among decaying substances; they are always minute in size, and like many other lower forms of life, are capable of retaining their vitality for a long period even when dried, which accounts for their wide distribution; this faculty is also possessed by certain of the parasitic Nematodes, especially by those which lead a free existence during a part of their life-cycle. The free living differ from the majority of the parasitic forms in undergoing no metamorphosis ”

“ At the same time, we see nematodes whose males are attached to the females, so as only to form a single individual; in other cases, the male seems to disappear to such an extent, that we find nothing but the male organ in the female; indeed, there are instances of male worms, which, without changing their form, occupy the cavity of the matrix and, like the lernean crustaceans, are parasites of their females. The Trichosomum crassicauda is an instance of this kind.
Arrangements which would not have been suspected beforehand, are every day revealed, with respect to the conservation of species. ”

“ Trematodes live free and active within the organs attacked, though they may attach themselves by suction for a longer or shorter period; in other cases, however, they bore more or less deeply into the intestinal wall with their anterior end, or lie in cysts of the intestinal wall which only communicate with the lumen through a small opening; in those species living in the lungs of mammals the host likewise produces a cyst, which usually encloses two specimens ”

“ It is found, that in Egypt, a distoma is developed in the blood, and occasions a very severe malady, scarcely known to physicians. In Iceland, a cestode causes the death of a third part of the population. Worms develop themselves in the eye, and may even cause blindness; the Cœnurus of the sheep causes giddiness, and becomes fatal to the animal which harbours it. The chlorosis observed in Egypt and Brazil must, it appears, be attributed to a considerable development of a nematode worm, which lives in the small intestines, and which naturalists know under the name of Dochmius duodenalis ”

“ History.— Our knowledge of this subject dates from the discovery by Demarquay, in 1863, of a larval nematode— microfilaria bancrofti (Plate X.) —in the milky fluid from a case of chylous dropsy of the tunica vaginalis. Later, in 1866, Wiicherer found the same organism in the urine of a number of cases of chyluria. In 1870 Lewis made a similar observation in India, and in 1872 discovered that the blood of man was the normal habitat of this larval parasite, which he named, accordingly, Filaria sanguinis hominis. ”

“ It would seem, in short, that the eggs of nematodes which normally take up their residence in cats, dogs, and in the carnivora which reside in arid regions, will develop embryos in the egg without external moisture. As before remarked, Davaine thinks it is not necessary that these nematode embryos should pass through any intermediary bearer, and he believes that they are often directly transferred to the stomach of their “hosts” whilst adhering in the form of an impalpable dust to the coats of their bearers, whence they are detached by the animal’s frequent habit of licking the fur. ”

“ Hermaphroditism (as in Trematodes, Cestodes, and a few Nematodes) is a further peculiarity of many permanent parasites; moreover, the association in couples that occurs, especially in trematodes, may lead to complete cohesion and, exceptionally, also to re-separation of the sexes. In many cases the females only are parasitic, while the males live a free life, or there may be in addition the so-called complementary males. Occasionally the male alone is parasitic, and in that case lives within the female of the same species, which may live free, like certain Gephyrea (Bonellia) ”

“ If any argument were necessary to show how desirable it is to furnish full descriptions of the commoner kinds of parasite, I could adduce numerous instances that have been brought under my notice where professional men and others have been entirely mistaken as to the essential nature of their parasitic finds. Thus, I have known an instance where a great authority on the diseases of dogs has persisted in asserting for the free proglottides of a tapeworm a nematode origin; and, in like manner, human tapeworm-segments have frequently been mistaken for independent fluke parasites. ”

“ Amongst the retrogressions, the loss of the organs of sense may be mentioned, particularly the eyes, which are still present, not only in the nearest free-living forms but also in the free-living larvæ of true parasites. It is only quite exceptionally that the eyes are subsequently retained, as a rule they are lost. Lastly, in a great many cases the digestive system also disappears, as in parasitic Crustacea, in a few nematodes and trematodes, in all cestodes and Acanthocephala. There remain at most the rudiments of the muscles of the fore-gut, but these are adapted to entirely different uses. ”