242 



ZOOLOGY 



ftfiOT. 



undergo a process of segmentation similar to the holoblastic 

 segmentation of the impregnated ovum, resulting in the 

 formation of a rnorula, which becomes converted into a stage 

 resembling a gastrula. The gastrula elongates and gives rise 



to a body called a redia 

 (C), which begins to move 

 about, and, eventually 

 forcing its way out of 

 the interior of the sporo- 

 cyst, finds its way to 

 some other part of the 

 Snail, usually the liver. 

 When fully formed, the 

 redia is a cylindrical 

 body with a pair of short 

 processes (proc.) near the 

 posterior end, and with a 

 circular ridge near the 

 anterior end. It possesses 

 a mouth leading to a 

 pharynx and simple sac- 

 like intestine, and there 

 is a system of excretory 

 vessels. In the interior 

 of the redia cells are 

 budded off and develop 

 into gastrulse, exactly as 

 in the case of the sporo- 

 cyst ; these gastrulse 

 either develop into a fresh 

 generation of redia3 if the 

 season should be winter, 

 or, if it should be sum- 

 mer, give rise to bodies 

 termed cercarice. The 

 latter (D) are provided 

 with long tails : they 

 have anterior and pos- 

 terior suckers, and a 

 mouth and pharynx, 

 followed by a bifid intes- 

 tine. An opening, the 

 birth-opening (C, b. op.), is formed in the wall of the redia 

 near the circular ridge, and through this the cercarise escape ; 

 they move actively by means of their tails, and force their 

 way out of the body of the Snail. They then, losing the tail, 

 become encysted, attached to blades of grass or leaves of other 



FIG 194. Tsenia solium. Entire specimen, 

 reduced ; cap. head. (After Leuckart.) 



