330 ANIMAL BIOLOGY. [Part II. 



col.) enabling it to fix itself while it browses. There is an 

 excretory system (not figured) consisting of two groups of 

 ciliated infundibula, one just behind the collar, the other just 

 behind the posterior processes. The mouth is surrounded by 

 lips, and there is a muscular pharynx (ph.) opening into the 

 digestive sac (di.), consisting of a blind tube lined with clear 

 nucleated cells. There is also a definite birth-opening (b. o.). 



At the posterior end of the redia, in the lining of the body- 

 cavity, there are large clear cells which undergo segmentation, 

 and form mulberry masses (morulce) enclosed in a delicate mem- 

 brane. The morula is detached into the body-cavity, and is 

 invaginated to form a gastrula, the walls of which are, however, 

 in contact, so that the primitive cavity (firchenteron) is obliterated. 

 Some of these would seem to develop into rediae (re.) like the 

 parent redia. Others become at first oval, and then more and 

 more elongated, whilst one end becomes narrower, and is partly 

 constricted from the remainder to become the tail of the cercaria 

 (ce.), while the rest of the tertiary embryo becomes its body. 



The Cercaria. A sucker appears at the anterior end of the cer- 

 caria (Fig. 97, G-., o. s.) y in the midst of which is the mouth-opening. 

 A second sucker (p. s.) appears about the middle of the ventral 

 surface of the flattened body. The digestive tract has a less 

 marked pharynx (ph.) than the redia, and is forked; but the 

 limbs have no lateral branches. When it is about -3 mm. long 

 the cercaria makes its exit from the redia by the birth-opening. 

 In mature cercariae there are very minute spines on the anterior 

 end of the body. Within the body, at the sides, are a number 

 of opaque white granules, forming the cystogenous organs (cijs.). 



The cercaria passes out of the intermediate host, and after a 

 short active life comes to rest on a blade of grass or other 

 object, assumes a spherical form, and is encased in a white cyst 

 composed of opaque white granules from the cystogenous organs. 

 The tail is pinched off during the process of encystment. 



The Cyst is thus round and snowy white ; but within it lies 

 the minute transparent embryo a tailless cercaria which, if it 

 pass into the alimentary canal of a sheep, will be carried thence 



