ANIMAL PARASITES 



15 



Within these are developed numerous elongate redice, with 

 simple, unbranched alimentary canal. These redise leave the 

 sporocyst and invade the liver of the snail where they give rise 

 to daughter redice or directly to the third form, or cercarice. These 

 are minute, tadpole-like forms with a bifurcate intestine, a tail, 

 and, in this species, two suckers. The cercarise escape from the 

 snail and, losing their tail, become encysted on grass and other 

 herbage. 



PRACTICAL WORK 



While Fasciola hepatica, the sheep liver fluke, occurs only in 

 limited areas in the United States, most of our vertebrate ani- 

 mals, from fish, frogs, and reptiles to 

 man, are infested by their own special 

 species of flukes which undergo essen- 

 tially the same cycle of development as 

 does the better known species from the 

 sheep. The larval stages of these flukes 

 are to be found in snails from our ponds 

 and lakes, sometimes two or more 

 species together. Study and make 

 drawings of the following stages: 



Eggs. — These may be studied in 

 prepared slides or by obtaining living 

 flukes from the lungs or intestines of a 

 frog, teasing them apart, and mounting 

 them in water. The eggs are ovoid 

 bodies with a smooth brownish shell 

 and a minute cap, or operculum, at one 

 end (Fig. 3). The contained proto- 

 plasm may be undivided or in varying 



degrees of segmentation depending on the species. When the 

 embryo develops, it pushes open the operculum and escapes as a 

 free-swimming miracidium. 



The miracidium (Fig. 4), is more or less conical, with a short 

 papilla at its anterior end. The surface is covered by cilia giving 

 the living organism a superficial resemblance to a rapidly moving 

 infusorian. An eye spot is usually to be seen a short distance 

 from the anterior end. A deeply staining mass posterior to the 

 middle of the body is made up of the germ cells. The miracidia 



Fig. 4. — Miracidium of 

 Schistosomam japan icum. 

 CG, cephalic gland; DE, de- 

 generate embryo; E, develop- 

 ing embryo ; G, gut ; LG, lateral 

 gland duci; N, nerve center. 

 (After Faust and Meleney.) 



