26 



ENTOZOA. 



of the cercaria, wliich is faithMly represented below (fig. 7). 

 Here, therefore, we have an oral sucker (a), a pharyngeal bulb (b), 

 an oesophagus (c), two ahmentary coeca (d, d), a ventral sucker (e), 

 a water-vascular system consisting of two excretory (urinary. 

 Van Beneden) ducts (g, g), and a contractile vesicle (/) by means 

 of which the ducts communicate with the external surface. The 

 tail (h) is largely developed, and characterized by a highly distinc- 

 tive marginal fringe {i, i), which has not, apparently, been foimd in 

 any other form of cercaria. Be that as it may, this free-swimming 



Fia. 7- — Fully developed cercaria of Bistoma militare. X 300 diam. — Van Beneden. 



tailed cercaria strictly conforms in its alimentary structure to the 

 general Trematode type, but before it passes into the sexually 

 mature condition it has other changes to undergo. These are 

 brought about, in the first place, by their parting with their tails, 

 and subsequently by their encysting themselves usually on or 

 within the surface of the body of some moUusk or aquatic insect. 

 This constitutes their pupa condition as shown in the an- 

 nexed illustration (fig. 8) where the pupa is represented in the 

 act of escaping from the cyst. The pupa itself differs from the 



