456 TEXT-BOOK OF ZOOLOGY 



its hooklets and suckers, the importance of which will now he evident. 

 (Why is this attachment necessary ?) 



2. A short distance in front of the posterior end of the knob, which 

 is now described as the "head " of the tape-worm, an annular groove is 

 soon after formed, a segment being thus formed, which, however, remains 

 in connection with the head. Shortly after a second segment is thus 

 constricted off, then a third, and so on, all of which segments grow 

 rapidly in size. In this manner is formed gradually a tape-like (name) 

 chain of segments from 7 to 10 feet long, which, together with the head 

 from which it has been produced, forms the tape-worm. Lastly, the 

 terminal segment, or -proglottis (i.e., the oldest of all the proglottides), 

 separates off from the chain, and is expelled with the faeces. The same 

 fate befalls the new terminal proglottis {i.e., the second oldest), and so 

 on ; and in proportion as segments are lost at the end of the tape- worm, 

 new ones (to the number of a thousand or more) are constantly formed 

 at the head, which, by absorbing food, continues to grow. 



3. The tape- worm lives on the nutritive fluid prepared in the stomach 

 and intestine of its "host." (Explain this term.) It thus obtains its 

 food in a digested condition, and hence does not need an intestinal canal. 

 For the same reason the beginning and end of such a canal, i.e., a 

 mouth and anus, are also absent. The absorption of food, in which, as 

 it were, the animal swims, proceeds, accordingly, over the whole surface 

 of the body. 



4. Hence this delicate .animal always obtains the moisture which is 

 necessary to its existence (see earth-worm, Section 1). 



5. The tape-worm, like most animal parasites which live in the 

 interior of other animals or in plants, is devoid of eyes and colourless. 

 (Give other examples.) 



6. The so-called "ripe proglottides" which have been separated and 

 expelled continue to live for some time after their discharge, but finally 

 perish. Their contained eggs — each proglottis has been calculated to 

 contain about 5,000 — are set free, and we have accordingly arrived 

 back at the point we started from, having thus completed the life-circle 

 in which the development of the tape-worm proceeds. Whether one of 

 these tough and resistant eggs (see Section A) is consumed by a pig, 

 which may be described as the "intermediate host" of the tape- worm 

 (explain this name), and whether the cysticercus, which may possibly 

 proceed therefrom, eventually arrives in the stomach of a man, are 

 questions of circumstance and accident. Among the enormous number of 

 eggs, amounting to many millions, produced by a tape-worm during its 

 lifetime, it is, however, quite probable that this will happen to some. 

 Thus we understand the large number of the eggs, which is only rendered 



