38 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



solid masses, morulce, each of which becomes a gastrula (i.e., a 

 two-layered embryo with a mouth and rudimentary digestive 

 cavity), by an inpushing or imagination of cells on one side. 

 This becomes a redia, which breaks through the wall of the 

 sporocyst, the ruptured place afterwards closing up. 



Redia (d). Body elongated and cylindrical. Near the anterior 

 end is a thickened ring or " collar," while not far from the pos- 

 terior end a pair of blunt processes project, one on each side. 

 The body wall has a structure similar to that of the sporocyst, 

 but the muscle is better developed. A mouth (0) is present at 

 the anterior end of the body, which leads into a muscular pharynx 

 (Ph). This is continued into a simple digestive sac (D). Branched 

 excretory trunks (Ex) commencing in ciliated funnels are pre- 

 sent. 



The redia wanders over the body of the snail and feeds upon 

 its tissues, especially the liver. The posterior processes act as 

 foot-stumps, and the collar serves as a relatively fixed point upon 

 which the anterior end can move when feeding. Within the 

 body-cavity, daughter ^redice, or the next, cercaria stage (c), may 

 be developed, the former only in warm weather. In either case 

 some of the epithelial cells lining the body-cavity enlarge and 

 segment to produce morulse, which become gastrulse, and grad- 

 ually assume the form of daughter-redia3, or cercarise, which 

 escape by a special birth-opening behind the collar. 



The Free Cercaria (e) resembles a minute tadpole. It has a 

 rounded flattened body armed anteriorly with minute spines, and 

 possessing anterior and ventral (S) suckers, and a muscular tail. 

 The body-wall contains numerous large granular lime-secreting 

 cells (cystogenous cells). A mouth, pharynx, gullet, and simple 

 forked intestine (D) are present, as well as excretory organs (Ex). 

 The cercaria makes its way out of the snail, loses its tail, and 

 attaches itself to a grass-stem, the cystogenous cells pouring out 

 a secretion which hardens into a bright white cyst, within which 

 it develops into a young fluke. If this encysted stage is swal- 

 lowed by a sheep, the gastric juice dissolves the cyst, and the 

 young fluke makes its way up the bile duct into the liver, there 

 becoming sexually mature in about six weeks. It is worthy of 

 remark that the adult sexual stage lives in the higher vertebrate 

 host, where it has a better chance of being preserved. 



