CESTOIDEA 



599 



The outer envelope and the shell are soon lost, and when the 

 embryo appears in the faeces it is surrounded by its inner envelope, 

 the embryophore, and has] developed six hooks. In this stage of 

 development it is called an onchosphere. 



It will therefore be evident that the so-called egg as seen in the 

 faeces is not an egg at all, but the onchosphere with its embryonic 

 envelope, which in certain species may be ciliated. 



When the onchosphere enters 



Fig. 243. —Free Ciliated Embryo Fig. 244.— Cysticercus of Tcsnia 

 OF Dibothriocephalus laius. saginata Goeze, 1782. 



(After Leuckart.) (After Leuckart.) 



throws off its envelope, and works its way into the tissues by its 

 hooks until it arrives in some suitable organ, when it throws off its 

 hooks, encysts, and forms either a little bladder-like cyst, from 

 the wall of which the scolex develops (the whole being called a 

 ' Cysticercus if the cyst is small, it is called a ' cysticercoid ' ; and 

 if in addition it has a caudal appendix, a ' cercocystis '), or it 

 develops directly into the scolex without the intervention of a 

 cyst, forming a plerocercoid (ttAv/p/?, full; KepKos, a tail). These 

 wanderings of the onchosphere in search of a suitable resting-place 



Fig. 245.— Plerocercoid of Dibothriocephalus latus. 

 (After Leuckart.) 



may produce unpleasant symptoms if there are a number of 

 parasites. The time occupied in the transformation of an 

 onchosphere into a Cysticercus varies, being from two to six 

 months, or longer. 



With but rare exceptions the Cysticercus does not develop 

 further until it enters another and different host, though some, 



