PLATYHELMINTHES 



161 



Fig. 87. — Left to right, Fasciola hepatica, 

 F. americanus, Dicrocoelium lanceatum; 

 natural size (drawn from author's speci- 

 mens) . 



After a period of embryonal development, which will only occur pro- 

 viding the eggs have reached water and suitable conditions of tempera- 

 ture, the larva escapes by the lifting of the operculum of the shell. It 

 is then in the stage of the miracidium (Fig. 88, 2), an infusorian-like 

 organism, ciliated, elongated, broader in front, and about 130 microns in 

 length. During its free-swimming period it must meet with a suitable 

 host within a few hours or it will perish. This host is a small snail, 

 usually of the genus Limnsea (L. 

 humilis) into which the larva 

 bores its way by a perforating 

 rostrum at its anterior extremity. 



If it escapes its aquatic enemies 

 during this free stage and arrives 

 at a suitable location within the 

 snail, usually the pulmonary 

 chamber, the larva loses its cilia 

 and digestive tube and becomes 

 transformed into a sporocyst (Fig. 

 88, 3) — a sort of reproductive sac, 

 ovoid in form and acquiring a 

 length of about 0.5-0.7 mm. The 

 cyst now becomes filled with germ-cells which are disposed in masses 

 (morula) ordinarily five to eight in number. 



The masses of germ-cells become transformed into so many redice 

 (Fig. 88, 5 and 6) which may be seen in different stages within the cyst. 

 The redise are cylindrical in form and have a simple intestine and 

 pharynx with lips turned out like a sucker. When they have attained 

 a certain stage of development the redise become actively motile, finalh^ 

 rupturing the maternal cyst aiid passing to another organ of the snail, 

 "usually the liver, in which location they grow to a length of 1.3-1.6 mm. 

 Within the body of the redia are germ-cells formed into six to ten cellular 

 masses which are to be 'transformed into so many daughter redise, or 

 directly into fifteen to twenty cercariw (Fig. 88, 7). Both daughter 

 redise and cercarise leave the mother redia by a birth-opening located 

 anteriorly. 



The developed cercaria has an oral and ventral sucker, a muscular 

 pharynx, and a bifurcated intestine which is as yet without lateral 

 branches. It has a flat oval body about 0.28 mm. in length, provided 

 "with a long actively vibratile tail. The cercarise escape from the snail 

 and swim about energetically in the water, eventually finding their way 

 to an aquatic plant or grass stalk. Here the tail is lost and the cystoge- 

 nous cells of the body form a mucoid substance which serves both to 

 encyst the cercarise and to attach them to the grass. The cysts may be 

 observed upon the specimens of vegetation as little white points about 



