THE TAPE WORMS. 



256 



worm, continues to live until the ova are expelled, it is an intermediate state. The scolex or head is 

 a kind of nurse ; it is asexual and buds metameres, which are reproductive. The eggs produced 

 within each of the metameres are numerous and too large to escape, except by rupturing the tissues ; 

 they may escape from the host, included in. the metamere when this becomes separated, and the escape 

 may be with the evacuations of the animal, or the metamere may move out by its own activity. 

 Under both circumstances the metamere has a power of independent movement, and creeps slowly from 

 the dung on to all kinds of moist substances, such as stalks of grass, leaves, and vegetables. In this 

 case they are eaten, with the vegetable matters, by vertebrates. Sometimes the metamere falls into 

 water, where it bursts, and the eggs are cast forth and are drunk by animals. Arrived in the stomach 

 of their new host, the metameres are more or less digested, and the eggs are diffused there, or some- 

 times imperfect digestion may occur, and the brood may reach the small intestine. In some instances 

 the eggs may be set free in the host by rupturing the metameres, and if they are then expelled 

 there is a chance of their being eaten or drunken and getting into the stomach and intestine of 

 a second host. The so-called eggs have tough shells, and the embryos within are totally unlike 

 their parents. They are globular naked vesicles, the largest being 0'05 of a millimetre in 

 length ; they have a cuticle, and either six, or four, microscopic booklets on their anterior ex- 

 tremity, with which they will, if they have the opportunity, bore inwards into the tissues of 

 their future host. 



These embryos are capable of motion, and under the influence of warmth and nourishing 

 juices around them begin to migrate. Each brings the central pair of booklets together like a 

 wedge, and thrusts and twists them into the mucous membrane of its host. The other hooks move 

 backwards, and finally the parasite reaches a small vein belonging to the portal system. By the flow 

 of blood it is carried into the liver and can go no farther. Or it may get into other blood-vessels and 

 be carried into the general circulation, and be deposited at last in some organ, such as the brain or in 

 the skin. The little vesicle with its hooks may grow in the blood-vessel, which may form a cyst 

 around it, or the parasite may penetrate the vessel and get into the tissues of the body, and a cyst 

 will enclose it there. 



Then a new growth occurs within the vesicles, and by a process somewhat similar to budding, one 

 or numerous bodies resembling the heads (or scolex) of a Tape Worm are developed. These, if they 

 escape by the death of the animal, or by its being eaten by others, will become Tape Worms in the 

 devourer. Thus the mouse, eating dirty substances, gets an embryo into its 

 body, and the cat eats mouse and embryo; and the new growths within the 

 last escape as the mouse is digested, and produce Tape Worm in the cat. 

 Or the part of the embryo which is covered with the hooks becomes developed 

 into a larger body, and has suckers and hooks differently arranged. 



The embryo from the egg, which thus becomes encysted in the tissues of 

 a vertebrate animal, is termed then a Cysticercus, and it forms one stage of a 

 disease, producing " measles " in pork, for instance. The Cysticercus being 

 swallowed by another vertebrate, the fore part of it, or the scolex, becomes 

 the head or asexual part o f a Tape Worm. Sometimes the cyst of the 

 embryo grows to a considerable size, and then the scolices which bud from its 

 inside are called Echinococci, and the cyst is a hyclatid. 



These Echinococci are, however, the product of a Tape Worm which 

 infests the dog and wolf, and the eggs, by some means or other, are swallowed 

 by men and animals and develop the truly dangerous hydatid disease of 

 the liver and other organs. This Tape Worm belongs to the genus Tzenia 

 and to the species T. echinococcus. 



Another species of Tsenia, or Tape Worm, which is the Beef Tape Worm,* 

 has the head without any coronet of hooks. It varies from fifteen to twenty- 

 three feet in length, and the metameres, some hundreds in number, have the sexual organs fully 

 developed in the 450th. The Cysticercus of this worm forms measles in oxen, and the scolex of 

 course has no hooks. The Beef Worm is found in man, and calculations have shown that it may 



* Tffnia mediacanellata. 



HEAD OF ECHINOCOCCUS. 

 (After Hiucley.) 



