ANIMAL PARASITES. 53 



c. After sticking in the finest vascular ramifications, enters anew 

 upon an active migration by means of its six small hooklels, passing 

 through the walls of the vessels into the neighbouring tissues, and 

 often continuing its wanderings for some time in closed cavities of the 

 body or in the soft parenchyma of organs, even when it has already 

 attained a certain size, so as to be visible to the naked eye. 



In the vicinity of the dwelling-place preferred by it the young 

 embryo becomes stationary, after the completion of an active, or an 

 active-passive, or an active-passive-active migration, and establishes 

 its habitation there, or wanders through the selected organ for a 

 time. By these active migrations, which undoubtedly take place 

 at a certain time, in the interior of the selected organ of the host, 

 the organ is irritated by the intruder, brought into an inflamma- 

 tory condition, and excited to produce exudations, which, by their 

 yellow colour, indicate the course of the wanderer, and form pas- 

 sages (streaks), either in the interior or on the surface of the 

 organ, which is also disturbed in its functions ; nay, if the 

 number of the immigrants be very large, or the organ selected of 

 great importance to life, the whole organism sympathises, and 

 functional disturbances, and more or less dangerous disorders are 

 produced in organs of particular importance to the individual. 



The question, what further takes place with the embryos when 

 they have come to rest in their new dwelling -placet leads us to 

 the consideration of the 



Third stage in the development of the Cestodea. This consists 

 of the so-called vesicular worms and their analogous asexual 

 forms, which, in accordance with the alteration which the em- 

 bryonal vesicle undergoes in the different species of tape- worms, 

 and with the different animals infested, may be divided into 

 cysticercal (vesicular worms), platy cereal (forms with a flat, inar- 

 ticulate, tail-like appendage), and acercal (or tail-less) forms. 



It is a common peculiarity of all the three forms here men- 

 tioned, that they are cestode-heads produced from the embryonal 

 vesicle, which occur still in the interior of the embryonal vesicle, or 

 upon it in a state of rest. We may therefore best comprehend 

 this step under the name of resting scolices. 



It is true that on account of the extremely small size of the 

 embryonal booklets in the species of Cestodea with which we 

 are here especially occupied, we have not yet succeeded in proving 

 the direct transition from the embryos to the resting, and usually 

 cysticercal scolices, by the re-discovery of the six embryonal hook- 



