186 THE AMERICAN MONTHLY [August, 



band, sometimes attaining a length of 16 feet, a breadth of a quarter of 

 an inch in its broadest part, and a thickness of nearly ^ of an inch. 

 The so-called head of the parasite is circular, not exceeding T 1 ¥ of an 

 inch in diameter, and is succeeded by a still narrower portion, the so- 

 called neck, from which the body enlarges to its posterior extremity, 

 which is its broadest part. Near the head, which is provided with 

 suckers by which the tape-worm attaches itself to the interior of the 

 intestines of its host, the joints or proglottides are short but gradually 

 increase in length until at the posterior extremity they attain a length 

 of ^ inch. Except as to its reproductive system the structure of this 

 tape-worm is simple. A digestive cavity is not required, since its 

 nourishment like that of some of the low vegetable parasites is derived 

 by exterior absorption. 



Tcenia mediocannellata is hermaphrodite and its reproductive sys- 

 tem is complicated and highly developed. Every proglottis is the 

 same as each of the other proglottides at the same age, for the youngest 

 proglottis is that nearest the head, while the oldest is the posterior, 

 the relative age of the intermediate ones being determined by their posi- 

 tion in the series. Here as in many forms of vegetable life Nature 

 prevents self-fertilization in that the male and female oi'gans of a pro- 

 glottis mature at different periods. In the mature proglottis the egg- 

 cavity has developed to such an extent as almost to exclude all the 

 other organs. This proglottis is practically a bag full of mature eggs 

 waiting to be detached and requiring only suitable conditions to de- 

 velop. The detached proglottis passes from the body of its host with 

 the faeces. In some species the detached proglottis maintains an in- 

 dependent life for some days, but in all it sooner or later decays, freeing 

 the eggs. During the life of a tape-worm there is a continued dropping 

 oft' of ripened segments and a formation of new ones by budding from 

 die head. 



Should the tape-worm be called an individual, or, like the Bryozoa, 

 a colony of individuals? Each proglottis is complete in itself and 

 identical with all the others except as to age. Tcenia mediocannellata 

 contains about 1200 segments, each capable of developing 30,000 eggs. 

 Seldom are more than 80 segments simultaneously filled with ripened 

 eggs, but if the whole 1200 matured at once an aggregate of not less 

 than 30,000,000 of eggs would be produced. The discharged egg of 

 the tape-worm contains in the interior of a capsule a little rounded 

 embryo with three pairs of horny hooklets at one end. 



Like the fluke the tape-worm requires to pass the intermediate stages 

 of its development in the body of some other animal than its final host. 

 In this species that intermediate host is the ox, while in another species, 

 T. solium, it is the pig. With its filth-indulging propensities it is easy 

 to see how the eggs of the tape-worm may enter the body of the pig 

 with its food, but as regards the fastidious ox it is less apparent. In 

 whatever mode the transference is accomplished into the alimentary 

 canal of the ox, the egg-case is dissolved and the liberated embryo bores 

 its way by means of its hooks through the tissues of its host and becomes 

 encysted in the muscles. When this beef is eaten by a human being, 

 the encysted embryo fixes itself by means of its suckers — already 

 present — and becomes the head of a tape-worm, the chain of proglottides 

 being produced by budding from its posterior end. 



