SCIENCE FROM AN EASY CHAIR.

Press, Volume XLIX, Issue 14668, 17 May 1913, Page 9

SCIENCE FROM AN EASY CHAIR.

(Bt Sis Rat Lankkstxs, K.CB FJR.S.)

MORE ABOUT.PARASITIC WORMS,

(Spedai rights »ecured by "Tho Press.")

The common tape-worm (as we have seen) lias man for ite "final" host, and the i>>g (or in some cases man) for its "intermediate" host. In tho "final" host it attains its full sizo and sexual maturity, producing millions of eggs, and fertilising them. In the "intcrniediato" host, in which it, passes its yrath. it is a bladder or cyst (also called a hydatid), wliich gives rise to a bud, provided with hooks and suckers, and ready to grip on to the wall of the. gut of its final host, and grow thero to great length and full maturity, should it have tho chance to be swallowed by the appropriate species, which is in this case a man. The Eehinoooccus tape-worm has, as I mentioned last week, the dog for its final host, and man and other animals for its intermediate host.

Tflcro are. other "pairs" which act as anal and intermediate hosts to Sti tereat specie* of tape-worms. Th£ one which m tie adult tape-worm condition Wia tho d jjf^. 0 ™ intermediator the shtepTin t * bram of which tho embrvo developed from tho tape-worm's egg, grows tho form of a largo bladder or cvst -ri 1 W» man f budded Jwok-aad-eucker heads ready to bo swallowed by a dog. It causes a disease known as "staggers" ux <h e sheep, ac Sng in junously on the brain, and so dJturbmg the control of the muLles of ,£V nOther *P-™« found l^j^ B fe it* intermediate host the rabbit. An interesting case is, i^L & PC ' yiOrm found in the rafs 2?n ? a " M,^ dders or CTsti in the internal "niu. (or proce£ses resemblin the pile of velvet characteristic of the small intestine of all mammals), without leaving the rat's body. This is an important caso as showing that the passage, from ono host to another of a different species, is not an absolute characteristic of tape-worms. It is probable that this rat's tapo-wonn carries out its life in tho more old-fash-ioned way, and that the other tapeworms aro descended from ancestors which had but ono host, although they have acquired tho advantageous habit of using a second or intermedial host, tho advantage consisting in n wider dispersal and better chance, of survival in new and more or ler-s ro mote districts, an advantage, which is gained by some parasites by leaving their- host and wandering freely in pond, or stream, or the sea. for a certain period of their lives.

Other pairs of hosts for tape-worms are birds and fish—the bird Borving as final host—tho fish as intermediate host being oaten by tho bird. Similarly a whole scries of most curious tapeworms are distributed amongst pairs of fish, a carnivorous larger fish being hnal host and preying on tho smaller rri. aro interm ed«ato hosts. Tho most interesting tape-worms, from tho point of view of an enquiry as to. whence tape-wonr.s came and what wero their ancestors,, are those in which tho segments of tho adult worm aro suppressed and the tape or band formed by tho adult is unjointed —all in ono continuous piece. A common one of this kind is known as "Ligiila," and is found in fishes and birds. Although it is not marked into "lengths" or "segments ,, or '"joints," its smooth, tapelike body contains a whole series or complete reproductive organs, each with its separato genital pore, and theso aro found to follow one another at equal distances as wo examino tho "tape" A considerable step further in simplification is shown by the two or three kinds of tapeworm, which aro quite short, more or less oval in form, and hare only ono set of reproductive organs. Theso certainly come nearest to tho ancestral worms which wero tho common stock from which tho tape-worms, the flukes, and tho free-living planarians arose. Tho commonest and most interesting of those is named "Caryophyllasus,'. , from the resemblance of that, end of its body which corresponds to the part bearing hooka and suckers in other tape-worms, to tho flower- of a "pink" or "clove." It lives in freshwater flshes (such as the carp) in its adult condition, and lias for its intermediate host , the common littlo rod river-worm, which resembles « very email earth-worm (about an inch and a _ half long), I first made acquaintance with those littlo river-worms, and consequently with the parasitic clove-headed tape worm, when, from old Hungerford Suspension Bridge, I saw the mud of i the Thames banks at low tide coloured for tho length of a niilo or more of a deep crimson, by (as my father told me) tho innumerable hosts of these littlo blood-red creatures. The common river-worm is actually blood-red, due to tho chrmical substance found' in bur own blood and called iiaeiuoglobiri, which thus colours theso worms to such an extent that you may have leaf-green and blood-red rsicle* by side as important features in a natural landscape. I was . a schoolboy then, and lost no timo in getting a pail and descending to tho muddy flats at the fine old water-gate of Buckingham Houso. which is now shut off from tho river'by the Thames Embankment. I drove home in a fourwheel cab with my pail full of Thames mud and red worms. I kept them, in a large earthenware dish under a slowly-running tap in a wonderful room at the back of my father's houso in Savillo row. Though capable of living in the foul mud and nourishing themselves on it, thoso swarming littlo worms were very sensitive to the supply of oxygen gas by running water? It is for tho purposo of absorbing and holding oxygen gae that they have the beautiful blood-red crystals dissolved in their blood-ves-sels. That seizure of oxygen is the special property of haemoglobin If I turned off tho.tap—in five minutes the worms became unhappy, ceased the regular waving movement of their tails projecting from tho mud, and' separated from it as miserable, wriggling littlo cork-screws.

I had a microscope and overythin" else a naturalist could desire, in that back-room"— tho paradise of wy boyhood—and examined, dissected, and drew hundreds of these little red worms, which are, by tho way, known as lubifex rivulorum-Tubifex, becmiso each makes a short volcano-liko tube of mud particles when undisturbed, and all is going well.. They laid their eggs and multiplied constantly. Every now and then I found ono looking rather plumper and paler at tho middle of tho body than its fellows (they are all somewhat distended in that region when ripo with eggs and sperm)—and, on carefully opening these fatter worms a most strange-looking little milk-whito parasite (about one-sixth of an inch ■ > )g) issued from the cut. It had a .spreading tbree-lobed ' "head," excessively mobile, changing shape, widening.; and narrowing, folding, and unfolding. It showed, when iooked at under tho microscope, the canal-system of a tape-worm well developed, but no digestive sac nor any mouth. It possessed immaturo germ and sperm sacs.

From the end opposite to the everchangins, clove-flower "liead" hung a curious hollow tail to the best of my recollection, motionless. On it I found three pairs of small bnstlos. or "hooks " irregularly scattered. This "tail'" with its six bristle-liko hooks, is tho six-hooked embryo to which the e<:°- of all tape-worms R i VPS rise, and "from wh:ch the tronja-heads, with their crown of hooks and suckers, are budded In this caw the pix-hooked embryo hnd becomo elongated and given ri?e, as :i a bud." to thß little worm, which lird grown much bippr than the six-hooked embryo now tTaihn R behind it as a tail. Eventually this tail atrophies altogether, and if the little river-worm in which it is lmne i S swallowed bv a fish—the younjr dove-hrad or CarvophylUmis remains in tho fish's intestine crows to be nearly threc-nuarters'of nn inch lons, and ripens and deposits fts eg«s. Tfhidi. passmß into the stream? are swallowed here and there hv n tnhi rcKl-nvor worm, and - develop within it into six-hooked From thecp the complete worm is budded m wo havo seen. '

Th5S known m °U i! mp!e fonn « f tapeworm known. It has not a!rrived i t tho stacp of develonina a - ei !J1 A % hooks and suckers, but the end'of tbl bmlv where they crow i,:!" 0 the worms is a wonderfully.SoWuT P<> " tiro organ, and SJ worm has not so irreatly ch-Tnw £ the form mA h-toA* </ *?*??*. m npn-parasitk ancestry. ' Ir ee-living,

The problems of "pa T fte,-f,w,, ecated by such .ertreaff • *-"«"y specialised

f creatures as the tape-worms—namely, i the questions.as to how they gradually lost their digestive apparatus, now tney developed their adhesive sUcKers and grappling hooks, and how they J managed to acquire tho double adaptation of structure and habit to two different hosts, one for their youth and another for their mature age are made somewhat less obscure by tho history of a very different class or parasitic worms, tho "thread-worms, "round-worms," or Nematodes. Theso are fi.no thread-liko crtstfures varj* in g in siso from' a fraction of an inch to six feet. They have an altogether peculiar etructuro unlike that of an J" other worms. They have a well-deve-loped mouth and digestive canal opening at ono end of the body, whilst tho j mouth opens at the other. They have a hard hornv skin, and are flexibio but firm and capable ef penetrating soft living creatures with tho sharp.ypointed end of their bodies. They are enormously abundant; there are hundreds of specie.* ct them, and as many are free-living and non-parasitic as aro parasitic. Many aro both —that is to say, pass a part of their _ lives as para- ! sites, and part as free-living worms. i But even those which are non-parasitic iive chieflv in animal and vegetable refuse or detritus (like tho plants which aro called ".saprophytes")., and many live on tho slime on the body of f molluscs and such animals, as it were j on the way to parasitism, but not yet j definitely committed to it. Tho species | of parasitic Nematode worms are moro numerous than all tho other internal parasites known. Two dozen species of them are known as parasites of man, some being very seriously injurious. 3lany are parasites of plants. Some have a single host, others have a final and an intermediate host; others are parasitic wheu young and free-living when mature, and.tho reverse conditions obtain in others. One of them has within tho last few weeks been credited, by a competent investigator, with an important part in the causation of the terrible disease., cancer. But hcip I must pause until another week gives mo space to te.'l of them.