WORMS.
By the name “Worms,” people commonly indicate a number of different forms whose relations with one another are by no means so close as those of a Holothurian and a Crinoid, or a Mussel and an Octopus. There are not, indeed, any common characters by the possession of which the worm-like animals can at once be distinguished from other animals. We take the divisions, examples of which are here represented, either by drawings, models, or specimens preserved in spirit separately.
The groups referred to may be enumerated as follows:—
| Platyhelmia. | Turbellaria. |
| Trematoda. | |
| Cestoda. | |
| Nemertinea. | |
| Nematoidea. | |
| Chætopoda. |
Platyhelmia, or Flat-Worms.—These form the lowest and simplest division of the group.
The parasitic Platyhelmia—the Tapeworms (Cestoda) and the Flukes (Trematoda)—occupy Case I.; the life history of the common Tapeworm (Tænia solium) is shown by the aid of models and figures. A model of the anterior end of the common Tapeworm shows the four suckers and the crown of hooks; the unjointed neck is followed by the joints (proglottids), which increase in size the farther they are from the neck. Several entire specimens of Tænia follow, showing the size of the whole worm and the form of its joints. The structure of the body is shown in the models of two joints. The growth and development of the Tapeworm is dependent on a migration or a change of the hosts which it inhabits in the various stages of its life; and although the different kinds of Tapeworm differ from each other somewhat in certain details of their migration and development, their life history exhibits, on the whole, the same events which we find in Tænia solium, a common Tapeworm of man in Northern Europe. This worm is matured in the intestines of man; its final joints consist merely of fertilized ova which have already passed through the earlier stages of development; when the joints are detached and discharged, their contents escape in the form of embryos contained in a thick chitinous shell. If these are now swallowed by a pig, the shell is digested by the gastric juices of the new host, and a rounded embryo, which is provided with three pairs of hooks, is set free; by means of these hooks the guest makes its way through the wall of the stomach or intestine, and finally settles down in the muscles of its host. The embryo now loses its hooks, and gradually acquires a bladder-like form, the central cavity of which is filled with fluid. This bladder-worm (Cysticercus) has its outer wall pushed inwards at the anterior end, and on this hooks and suckers become developed. We have now a narrow head and neck with an attached bladder, the head being at this time hollow. If during the long time that these bladder-worms remain alive, the pig is killed for food, its flesh is found to be “measly”; if it is afterwards insufficiently cooked and eaten, the worms are conveyed into the human stomach. Here the bladder-like termination becomes absorbed, and, the neck beginning to grow, we have the commencement of the form from which we started, and the completion of that “vicious circle” which is so curious a characteristic of many forms of parasitic life.
Fig. 8.
Tænia solium: showing the head (h) with its suckers (s′) and crown of hooks (s), the unjointed neck (n), and a few of the succeeding joints (j).
In other Tapeworms the cyst may be more complicated than that in the pig, as, for example, the form found in the sheep’s brain or the liver of the horse.
Of the other Cestode parasites mention should specially be made of those of Fishes; the vulgar notion that the parasites of these animals are dangerous to man has been shown to be entirely erroneous.
Fig. 9.
Limnæa truncatula.
The Flukes infest animals of all kinds; that which is most dangerous to sheep, and the cause of much pecuniary loss (Distoma hepaticum), is selected here as a type; its structure is shown by a large model, and its life history by a series of diagrams (Figs. 10–13). Here, again, we have a creature which infests two hosts. If the larvæ which escape from the sheep fall on wet ground in or near a pool, they make their way to a small pond-snail (Limnæa truncatula, Fig. 9), into the lung-chamber of which they bore their way. On leaving them the larva may be, and is, too frequently, eaten by a sheep, and makes its way into the liver of that animal, where it causes the disease known as the “liver rot.”
The damage done by the liver-fluke may be imagined from the fact that in the winter of 1879–80 no less than three millions of sheep died of rot in the United Kingdom; this heavy loss is no doubt largely due to the immense number of eggs to which a single fluke may give rise. It has been estimated that every fluke may produce, during its life, several thousands of eggs; and in one case Prof. A. P. Thomas found as many as 7,400,000 eggs in the gall-bladder of a sheep which was suffering from rot, and which, at that time, had in its liver about 200 flukes.
The non-parasitic Flat-worms are shown, magnified, in the upper parts of Cases I. & II. The Turbellaria proper, without any or with a simple or a branched intestine, but without a vent, are represented by Convoluta and Thysanozoon: the general structure is shown by a diagram in Case II., which is here reproduced (Fig. 14). Planaria, Thysanozoon, and Bipalium serve to illustrate the forms of members of this group.
The Nemertine Worms (Nemertinea), with a straight intestine, with a vent, and with a proboscis, may attain to a very considerable length; Carinella and Lineus are represented by large figures, and various species are shown in spirit. These forms, which used to be very unsatisfactory to exhibit, on account of the great difficulty of preserving them complete and uninjured, are now, with improved methods, very satisfactorily shown, as the specimens purchased from the Marine Biological Laboratory at Plymouth prove.
Stages in the life history of the Fluke.
Fig. 10. Egg of Fluke, showing the operculum and the contained yolk-spheres. Magnified 340 diams.
Fig. 11. An embryo forcing its way by its boring-papilla (p) into the wall of the lung of a Snail (e.p.) Magnified about 340 diams.
Fig. 12. A young Rédia (natural size, ½ millimetre or ¹⁄₅₀ inch): pl., pharynx; g, contained germs; p, characteristic posterior processes of the Rédia.
Fig. 13. Free-swimming Cercaria, before the commencement of the formation of the cyst. Magnified 100 diams.
Fig. 14. Diagram of the structure of a Turbellarian: ng, nerve- (cerebral) ganglia; nb, nerve-branches; yg, yolk-glands; t, testis; o, ova; ov, ovary; c, cirrus; m, mouth; ph, pharynx.
Fig. 15. Diagram of a Nemertine: b, brain; m, mouth; n, renal organs; id, diverticula of intestine; g, gonads; sn, side nerve-trunk; pr, proboscis in its dorsal sheath.
Fig. 16. Diagram of the structure of a Nematoid; m, mouth; ph, pharynx; a, anus; o, orifice of genital tube.
Nematodes (Thread-Worms or Round-Worms).—These are for the most part parasitic, and infest plants as well as animals; the common Round-Worms living parasitically in man (Ascaris, Stronaylus, Trichocephalus) belong to this Order. Sometimes they are parasitic in their early stages and later live a free life—such are Gordius and Mermis. A specimen of a Mantid is exhibited from which half the body of the infesting Gordius has already protruded (Fig. 17). One of the most remarkable Gordii is the great elongated G. fulgur, or “Lightning Snake,” from Celebes. Another very large Nematode is the so-called Guinea-worm, or Dracunculus medinensis, which is found beneath the skin of the leg; it is very possible that this worm was the cause of the illness which afflicted the Israelites in their journey through the desert from Egypt to the Promised Land.
Fig. 17.
Gordius escaping from a Mantid.
Fig. 18.
Figure of Trichina spiralis, showing the worms encysted in muscle.
Of all Nematodes the most dangerous to man is the small worm which is known as Trichina spiralis (Fig. 18); a series of models are shown which give a good idea of the structure of the female and the smaller male. The young make their way through the walls of the stomach of their host, and encyst themselves among its muscles: a piece of a sternothyroid muscle is shown, taken from a man in whose body it was calculated there were forty millions of encysted Trichinæ.
Other Nematodes infesting man, such as Filaria sanguinis hominis, are too small for exhibition.
Plants are not free from the attacks of Nematodes, and examples are shown, accompanied by an illustrating figure, of the Ear-cockle gall of wheat; this gall is due to the injuries inflicted by a minute Thread-worm—Tylenchus tritici. Wheat is, of course, by no means the only cultivated plant that is attacked by these minute worms; the history of most has, however, still to be made out.
Holding a somewhat uncertain position in relation to the Round-worms are the parasitic Acanthocephali (Thorn-headed Worms) and the free-swimming Chætognatha, or Bristle-jawed Worms; examples of both of these groups are shown, together with diagrams illustrative of their general structure.
Annulata or Chætopoda.—So-called because consisting of a series of rings, and being provided with chætæ or bristles; they are to be associated with the Arthropoda, under the one head “Appendiculata,” a better name than “Articulata,” since Cuvier did not include worms in his group. The creatures that are most familiarly called worms are to be found in Case III.; here are a few examples of the numerous kinds of worms that are found living freely in the sea, of earth and freshwater Worms, and of Leeches. All these worms are distinctly characterized by the fact that they consist of a number of definite rings (somites), whence they have been called Annulata. The marine Worm and the Earthworm differ from the Leech in that these rings are provided with chætæ or bristles, of which there are a number in each bundle in the marine, and a few only in the terrestrial or freshwater form: hence the marine Worms are called Polychæta and the latter Oligochæta.
The former are divisible into two great groups. There are those that are free-swimming and are able to forage for themselves, such as the lovely Sea-mouse (Aphrodite aculeata), the large Eunice gigantea, the common Nereis pelagica, or the exquisitely coloured Chloeia flava. Others live a more retired life, dwelling in tubes, which they fashion for themselves; they lead either a solitary or a social life. Here we have examples of Sabella, Sabellaria, Serpula; a number of forms of worm-tubes, showing their great variety and beauty (see especially the delicate Filograna), are to be seen in the small Table-cases placed against the north wall of the Gallery. Attention should be especially directed to Mr. A. T. Watson’s beautiful preparations of Terebella littoralis. We give a figure (Fig. 19) after a drawing by that gentleman of the home of Panthalis oerstedi, the tube-forming habits of which have been carefully observed by him.
Fig. 19.
Home of Panthalis oerstedi.
Fig. 20.
Section across the body of an earth-worm to show the disposition of the more important organs; the body wall (w) consists of dermis, circular, and longitudinal muscles; the body cavity is divided by membranes (c) into a series of chambers, in each of which opens the mouth of a coiled nephridium (n). The axis of the cavity is occupied by the intestine (i); above and below it is a longer blood-vessel (v), and below it is also the central nerve-cord (nc).
The Oligochæta are represented by the common Earthworm, the influence of which in the formation of mould and in the general ploughing of the soil was carefully investigated by Mr. Darwin; and by the little Tubifex rivulorum (Bloodworm), which owes both its red colour and its ability to dwell in mud, which is so poor in oxygen as to be unfit for respiration, to the same chemical compound as that which gives the red colour to our blood and carries the oxygen of respiration all over the body.
Acanthobdella: e, eyes; ch, chætæ; s, sucker.
The Hirudinea, or Leeches, are often said to be distinguished from the Chætopoda by the absence of bristles, but, as a fact, Acanthobdella (Figs. 21 and 22) has very well marked bristles. They always have a sucker at the hinder end of the body by which they are attached to their prey; they are found in fresh water (Piscicola), on sea-fishes (as Pontobdella), or in moist places, as the Leech (Hirudo). The last-named has three jaws, armed with as many as ninety denticles. Trochetia subviridis (Land-Leech) is a species which is found rarely and sporadically in England.
The Myzostomaria form a division of Polychæta all the members of which live parasitically on Crinoids, and otherwise present great differences in their habits. Some move about freely on the Crinoids they infest, others are more sluggish and rarely move, others produce galls or cysts on their host, and yet others are internal parasites, and live in the alimentary canal. It is of interest to note that there are corresponding degrees of difference between the young and old specimens of the different groups of species.
The general organisation of Myzostomaria is shown in the accompanying figure (Fig. 23) in which the dorsal wall of the body is supposed to be transparent so as to allow of the chief internal organs being seen.
Fig. 23.
Diagram of Myzostomum to show the general form of the body and the marginal extensile cirri (c); within these and on the ventral surface are four pairs of suckers, and more internally five pairs of appendages each bearing two hooks; the proboscis (p), the digestive tract and its ramifications, and the reproductive organs are outlined as if seen through a transparent wall; a, anus.
The last group of Worms here represented is that of the Gephyrea; with the advance of our knowledge it is probable that they will be found to be more intimately allied to the Annulata than is now generally supposed; it will be seen indeed that Echiurus has bristles at its hinder end; Sipunculus is the best known representative of the unarmed Gephyrea; Bonellia is interesting both from the fact that it owes its green colour to a matter closely resembling the chlorophyll of green plants, and from the possession by the female of a proboscis, which is protruded from the hole in the rock occupied by the worm: the male is very much smaller than the female, and is not nearly so well developed. Owing to the mode of lighting the Gallery, the visitor may have to shift his position several times before gaining a good view of the whole length of the proboscis.