1920] Johnson: Life Cycle of Eehinostoma Rcvoh(tu»i (Froelich) 347 



Generally the length of the body and tail are about equal when moder- 

 ately extended, each being about 0.50 mm. The width of the tail under 

 average extension is about 0.05 mm. at the widest point. Actual 

 variation in the size of cercariae is not observable, the difference being 

 due entirely to the degree of contraction. The ventral sucker is some- 

 what larger than tbe oral sucker ; the former measuring on the average 

 0.094 mm. and the latter 0.072 mm. When seen from the side the 

 ventral sucker is often to be found greatly extruded (fig. 44). It is 

 located just back of the middle of the body. 



The number of collar spines is probably always forty-three. These 

 spines have been counted on a large number of cercariae and that 

 number seems to be specific. Although the possibility of missing count 

 by one is easily understood by those who have attempted such counts, 

 I feel quite safe in saying that the collar spines are forty-three in 

 number. The ventral and lateral spines of the collar are from 0.016 

 to 0.018 mm. in length, being a little larger than the dorsal spines 

 which are 0.014 mm. long. The spines are arranged in two alternate 

 rows, although the ventral ones show more of a clumping than of an 

 alternate arrangement. ■ These collar spines do not appear until per- 

 haps the very last period in the development of the cercaria. Often 

 apparently mature cercariae were found within the redia with these 

 collar spines either absent or faintly developed. This probably 

 explains why cases of echinostome cercariae have been recorded lacking 

 the collar spines. 



The surface of the body is partially covered with spines which 

 extend dorsally just a short distance back of the oral sucker. Ventrally 

 they reach well past the ventral sucker where they gradually loose their 

 definite size, shape, and arrangement. Figure 45 shows these spines 

 as arranged on the side of the cercaria between the oral and ventral 

 suckers. They measure in this region 0.006 mm. in length. 



The digestive system (fig. 41) is quite easily seen and of con- 

 siderable size, the intestinal caeca extending nearly to the posterior 

 end. The mouth is slightly ventrad, the typical echinostome position. 

 The pharynx is 0.035 mm. in length by 0.025 mm. in width, and is 

 preceded by a prepharynx about 0.028 mm. in length. The oesophagus 

 under average extension is about 0.14 mm. long, while the bifurcations 

 of the intestine are about 0.23 mm. in length. 



The movement of the cercaria is accomplished in two ways, one of 

 which is much slower than the other. The slower movement is leech- 

 like and is used only on a surface. This is the only movement possible 



