128 



463, demonstrate no periodicity or law by which future outbreaks may 

 be suspected or predicted. 



Sheep-owuers aud veteriuariaus are agreed that damp, wet seasons, 

 and damp pastures are favoring conditions for the development of the 

 parasite and promotion of the disease. 



The life history of the parasite has been determined by Leuckart in 

 Germany, and a little later, but apparently independently, by A. P. 

 Thomas, in England. The former published his observations in Zoolo- 

 gischer Anzeiger, December 12, 1881, and October 9, 1882, and the lat- 

 ter in the Journal of the Royal Agricultural Society of England^ Vol. 

 XVIII, part 2, 1882, and Vol. XIX, parts 1 and 2, 1883. These authors 

 have described most of the stages in the life history of the parasite, 

 leaving but little to be said in addition. 



The iig<^ of the fluke passes from the biliary passages through the in- 

 testine to the ground (Plate XVI, Fig. 2a). Those that fall in favorable 

 places develop (Fig. 2h) and finally break the little lid oft' the end of 

 the shell (Fig. 2o) and escape. This happens in summer and occupies 

 from three to six weeks. At this stage (Fig. 3) the whole body is covered 

 with fine cilia (hairs), which enable the embryo to swim about iu the 

 water. At one end of the embryo is a little projection which can be 

 thrust out aiul withdrawn, and is the apparatus by which it bores into 

 its second host. If it does not meet one in a day or two it dies. If it 

 meets a water snail (Fig. C and 6a), it is not slow to penetrate into the 

 body, where, according to Leuckart, it lodges in the respiratory cavity. 

 Here it encysts itself (Fig. 4), contracts into an oval mass, and rapidly 

 grows. The name sporocyst has been applied to this form. The con- 

 tents of this sporocyst split up into a number of bodies (Fig. 4), usually 

 from five to eight, which develop into redicc (Fig. 7). Their length 

 at this stage is about 2'"'", or one twelfth of an inch. These are ex- 

 cluded from the sac one by one through a rent. Now each redia, in 

 its turn, develops from fifteen to twenty cercariw (Fig. 8) within it, which 

 are evacuated in turn through an unpaired orifice situated under the 

 neck of the redia. The cercari?e are the forms that escape from the 

 snail, and are scattered by it in its wanderings. The cercaria, after a 

 time of active life, loses its tail, which it has developed, aud again en- 

 cysts itself. (See Figs. 11 and 12 and Fig. 10.) The contents of the 

 cyst still more resemble the future fluke, and it is at this stage that 

 the sheep swallows it with grass. It then breaks from the cyst, arrives 

 at the stomach and duodenum, to finally make its way into the biliary 

 canals and grow into another adult hermaphrodite, capable of giving 

 rise to other generations of young. 



According to Thomas the encysted embryo (Fig. 4) may give rise to 

 daughter redias or to cercarise, the former to develop in the summer aud 

 the latter during the cold season. 



The epitomized life history is first the egg j second, the embryo, which 

 encysts iu a snail j third, five to ^igUt yediae, developing from tlie oystio 



