MAN AND HIS ENEMIES.

New Zealand Herald, Volume LXVI, Issue 20171, 4 February 1929, Page 6

MAN AND HIS ENEMIES.

PA If A SITES OF TIIE BODY.

THE FLUKE AN EXAMPLE.

COMPLICATED LIFE HISTORY.

TWO HOSTS ESSENTIAL.

No. XVIII

iiy w. n. M'onEnon.

Fmm llin day that arboreal man descended from tlio trees and commenced to loam the long and painful lesson of walking creel upon two legs, his body lias btfon tho proy of a host of worm parasites. li is highly probable thai, tlio loss of Ibe hair that onco thickly clothed liis body was but an unconscious adaptA , iil-ion to tlio ravages of theso parasites.

Tlit:to aro many ways in which parar%ilic worms injure tlio body of the host. They may fiimply help thonislevs to the raw food materials which the host provides for bis own nourishment; but more often -they prefer that the host should also digest if., so that they may absorb their (ill at little trouble and at no oxpenso fo themselves. If these robbers ■«re numerous they may starve his body to the point of anemia, so that lie has but little resistance to the bacterial and oilier pathogenic organisms that invade bis tissues. Frequently, however, parasites feed upon the vil.al fluids of the host's body, upon tho blood and the lymph'j and then indeed their depredations are of much more serious character In their migrations, too, from one part ol the body to another, they often extensively damage organs ana tissues. Bomotimo:! they originate cystic: growths which, by the displacement or injury of vital parts, produce fatal effects, Initially many of theso invading organisms produce toxic matters which may cause widespread poisoning and consequent disease when liberated into tho blood of tho infested animal.

Tho Adult Ltvor Fluko. A considerable number of tho parasi!io worms Hint, infest tho body of mini mid his domestic animals belong to tho lowest of tho si wo nil great groups of -worms, that which is known us (lie flat ■worms. Of thoso there uro two importunt types to bo considered, tho flukes mid the tape worms. Tho flukes are best exemplified by tho common livorfluko of tho shocp, tho pnrasilo that is responsible for tho widespread and destructive "liver-rot" which, in marshy, ill drained regions, is tho bano of tho fheepfarmer. Tho adult fluke, which inhabits tho larger bile ducts of tho sheep in great numbers, is a thin, flattened, leaf-like creature, typically somewhat less than half an inch in length. Packed tightly into the bile ducts of tho sheep tho flukes feed and grow until maturity is reached, at which period tho reproductive organs bocomo complete and functional. Tho fluke is hermaphrodite, that is to say, each individual possesses a completo set of both mulo and femalo reproductive organs, tho . arrangement of all these essential structures, together with their Severn!' accessory glands, being decidedly complicated. When the eggs havo been fertilised, each is enclosed in a strong oval capsule of chitin, a somewhat hornlike material. Passing from tho body of tho parent fluko these eggs find their way ilito tho intestine of tho sheep, whonco they are passed out on to the grass. Tho. Fluke's Llfo-History.

After*ft time tho tiny lid with which tho egg capsule is fitted lifts off, and a larval creatine, too small to bo seen with tho naked eye, emerges. Jts body is covered with long, clelicato processes •formed cilia, by the rythmical movement of which it i« able to swim in water or to wriggle over tho surface of wet vegetation. This larva dies in a very few hours unless it is fo fortunate as to discover and attach itself to tho body of a small fresh water stinii. Tho snail habitually selected ' is a misiuto creature belonging to tho genua Limimoa, species of which are very widely distributed over tho whole world. .The tiny lacvh seems to be capable of dofooting. the presence of these snails in its neighbourhood, for it has boon known to actively follow along the snail tracks in the sliuiy mud along tho water's edge. Having successfully attached itself to a suitable snail the tiny creature commences to bore its way into the body of its new host, using its sharp, protrusible head-lobo for the purpose. Tho snail, being an air-breather, has a large pouch-like respiratory chamber, in the walls of which large blood Vessels are massed. In these blood spaces the larval worm at last conies to rest, and settling down, proceeds to undergo a remarkable series of changes, (gradually losing its distinctive characters it becomes transformed into an elongated sue like structure, in the wall of which there are certain germinal cells that produce tiny buds that pass into the central cavity. During tho warm weather these buds grow into little creatures which nro .distinguished as Cercariae. These are really young flukes, nit hough as yet they bear little resemblance to the adult form. The Cert'aria has a month and a simple put, but there is no trace of reproductive organs. / Tt soon breaks its way out of the parent, sac. and after a period of rest in the liver of the snail, it bores its way out ot that animal's body. It has a long tail, with the aid of which it is able to swim or (o wriggle over tho surface of wet herbage. After following a free living existence for a time, the Oerearia eventually settles down on tho grass, loses its fail and secretes a firm shell of chitin about its .soft body. In this encysted condition it can abido with little change for a long time, but unless it is so fortunate n.s to pass.into tho body of a sheep, it, is unable to complete its dual development. A Continued Parasite. The fluko' •has become so completely iidaptciMo its parasitic mode of life that ii i.s absolutely dependent on its various appropriate hosts at. different periods of its history. If, perchance, tho grass upon which is rests is- swallowed by a grazing sheep, the shell or capsule is rapidly dissolved by the digestive juices of tho sheep, and tlie young fluke is liberated in the stomach. 3t makes its way at once into one of the large bile ducts, where, taking (inn hold with its sucker, it proceeds to feed and to gn.w. When maturity is reached it. develops sexual organs and reproduces in/its turn. This . life.history reveals a very curious phenomenon not uncommon among lowly animals. In its course we see an example <>f what is called alternation of generations. Wo/have seen that the adult tluke develops dual sexual organs. The eggs are fertilised in the usual way before being enclosed in tho capsules. Clearly, then, (he lava which hatches out. of this egg is t hi product ot a m Mm) reproductive plot ess. Hut this larva becomes transoimetl into » sac which produces many J warm«, really young (hikes, by a proi n ti K Wl,i ° h H It. Nt of |„, vogot at no reproduction of asexual i' | S r * Knmv " «» asexual i eproduction. Than w» have tho Of" rtuko'7' • ** mn * ion H I t ° s followed by »I 1 asexual generation, multiplication Ink >U* phue_ in both phases. " ' jileiUMi of this complicated life-cycle two ,h< t animals are essential. First comes fe "» whuh the tluke complete* <ht. process of gnnvth and sexual reprothio lum. rdter which a fresh water S of cSC? T nm b0 Caniwl <0 * ~,m"