Chap, xiv.] DEVELOPMENT OF T^ENIA. 545 



further history is as yet unknown. The dorsal buds 

 take on the form of the parent with sexual organs, 

 but do not themselves become sexually mature ; they 

 develop a stolon from their ventral surface, on 

 which appear buds that grow up into the sexual 

 forms. 



The relations of these different stages is shown by 

 the following table : 



Sexual generation. 



First asexual form with dorsal stolon. 



Spoon-like forms developed Second asexual forms developed as 

 as lateral buda (future median buds with ventral stolon, 

 history unknown). | 



Sexual generation. 



A somewhat different condition of things is found 

 among the endo-parasitic forms, where, as a rule, 

 the animal passes through its different stages in two 

 different hosts ; we may take as typical the histories 

 of the common tapeworm, and of the liver-fluke which 

 causes the " rot " in sheep (Distomum hepaticum). 



Taenia solium is sexually mature in the intes- 

 tine of man, and the final joints of the tapeworm 

 consist merely of fertilised ova, which have al- 

 ready passed through the earlier stages of develop- 

 ment ; when the joints become free and escape to the 

 exterior, they break up, and the contents escape in 

 the form of embryos contained in a thick chitiiious 

 shell. If these are now swallowed by a pig, the 

 shell is digested by the gastric juices of the new host, 

 and a rounded embryo, which is provided with three 

 pairs of hooks, is set free ; by means of these hooks 

 the guest makes its way through the wall of the 

 stomach or intestine, and finally settles down in the 

 muscles of its host. The embryo now loses its hooks 

 and gradually acquires a bladder-like form, the central 

 cavity of which is filled with fluid, while circular and 

 JJ 16 



