324 PARASITES OF ANIMALS 



dogs, rabbits, and other animals, as well as by man himself, the 

 freed ova are dispersed and carried to considerable distances ; 

 and thus it is that a considerable proportion of them ultimately 

 find their way into ponds, ditches, canals, pools of all kinds, 

 lakes, and running streams. 



7. At the time of their expulsion the eggs exhibit a finely 

 segmented condition of the yolk. The egg-contents continue 

 to develop whilst outside 'the parent's body, the granular matrix 

 finally becoming transformed into a ciliated embryo, which 

 when set free follows the habit of infusorial animalcules in 

 general by swimming rapidly in the water. The escape of the 

 embryo is effected at the anterior pole of the egg-shell, which 

 is furnished with a lid that opens in consequence of the action 

 of prolonged immersion, aided by the vigorous movements of 

 the contained embryo. 



8. The ciliated, free-swimming embryo, at the time of its 

 birth, exhibits the figure of an inverted cone, its anterior extre- 

 mity, which is broad and somewhat flattened, supporting a 

 central proboscis-like papilla. A small pigment spot placed 

 dorsally, and having the form of a cross, is supposed to be a 

 rudimentary organ of vision. After the lapse of a few days the 

 cilia fall off, the embryo then assuming the character of creeping 

 larvae (planulae). 



9. Notwithstanding its abridged locomotive powers the non- 

 ciliated larvae sooner or later gain access to the body of an 

 intermediary bearer, within or upon whose tissues it becomes 

 transformed into a kind of sac or sporocyst. In this condition 

 the larva is capable of developing, agamogenetically, other 

 larvae in its interior. The sporocysts are highly organised, 

 forming redice. According to Willemoes-Suhm, the redia of 

 Fasciola hepatica lives on the body of Planorbis marginata. 

 This organised nurse, which is about a line in length, is the 

 Cercaria cystophora of Wagener. The progeny of this redia 

 consists of armed Cercariae, which after a time quit the nurse 

 to pass an independent existence in the water. 



10. In the cases of some species of fluke there is reason to 

 believe that before the Cercarice gain access to their final or 

 definitive host they re-enter the bodies of the mollusks. This 

 they accomplish by means of a boring apparatus, and having 

 previously cast off their tails they encyst themselves beneath 

 the surface of the skin. In this new situation they develop 

 into the so-called pupa, which is at length passively transferred 



