DEVELOPMENT OF TAPE-WORMS. 315 



less than in those Treraatoda whose history has been investigated. 

 In their mature state, when they are known as Tape-worms (Fig. 

 635), these creatures form an elongated strap-like body, divided 

 into numerous segments, with two longitudinal canals running 

 down the sides of the body, and united in each segment by a 

 transverse canal. At the anterior extremity, which is much 

 narrowed, there is a sort of head furnished with sucking discs, 

 and usually with a double crown of hooks, by the agency of 

 which the parasite is firmly anchored to the intestine of its 

 victim. The Tape-worm in this form is not however to be re- 

 garded as an individual animal, but as a colony ; each segment 

 being an individual in the ordinary sense of the term, produced 

 by gemmation from the head, and gradually removed to a greater 

 distance from the point where it was first formed, by the constant 

 production of similar segments in the same way. Each segment 

 is in fact a complete hermaphrodite, and, when mature, contains 

 an immense number of eggs ; on arriving at this condition it is 

 cast off from the colony, acquires a power of locomotion, and 

 quits the intestine of the animal which has hitherto harboured it, 

 in order to distribute its eggs in situations whence they may be 

 introduced into the bodies of other animals. With but few ex- 

 ceptions, however, the animals adapted for the early development 

 of the embryos of the Tape-worms, are not the same as those in 

 which the mature parasites are met with ; the latter are always 

 found in carnivorous, and the first products of development princi- 

 pally in herbivorous animals, not in the intestinal canal, but in the 

 closed and parenchymatous organs of the body. A s the mode of de- 

 velopment of these parasites is exceedingly curious and interesting, 

 and a knowledge of it is of the highest importance in medicine, we 

 shall give a short abstract of its leading peculiarities. The seg- 

 ments, which are now called proglottides,, when thrown off from the 

 mature Tape-worm disseminate their eggs in various places, as 

 they crawl about upon the ground or on plants. These eggs are 

 swallowed by herbivorous animals with the grass or other plants 

 which constitute their food, and on reaching the stomach the 

 little embryos escape from them. The embryos are small vesicles, 

 furnished with six minute hooks or spines, by the agency of 



