1920] j ii nson : Ljf c Cycle of Echinostoma Bevolutum (Froelich) 345 



dorsally or ventrally. Many other shapes may be found, however, 

 such as nearly spherical, very long aud slender, or even constricted 

 in one or more places. The characteristic echinostome marks are 

 present in all, however, if viewed from the proper angle. The 

 paired posterior protrusions are usually located near the middle of 

 the body. They extend ventrally, slightly laterally, and protrude 

 about one-half the width of the redia. They are rounded and non- 

 muscular, apparently never moving in a forward direction. Upon 

 these protrusions the redia usually rests when placed in water, the 

 anterior end often swaying from side to side. At the anterior end a 

 circular projection or "collar" is found. This "collar" is somewhat 

 rigid and into it the neck of the redia may be partially withdrawn. 

 Figures 24 and 25 show the elongated and contracted anterior end of 

 the redia. Just behind the "collar." to the left, to the right or dorsally 

 the birth pore is located. It is usually quite prominent. The mouth 

 is located at the anterior end, immediately followed by a pharynx 

 varying greatly in diameter as a result principally of the sizes of the 

 rediae. However, the average diameter is about 0.08 mm. The blind 

 intestine is club-shaped and about twice the length of the diameter of 

 the pharynx. 



The color of the redia varies from almost a transparent condition 

 to a heavy dark brown. The former condition is only to be seen in 

 young or active rediae, the latter usually in inactive and apparently 

 very old rediae. The light orange or light brown color, which is the 

 more common, is to be found in both the large and small, the young 

 and old. the active and inactive rediae. 



The activity, as stated, varies much, the greatest being in the 

 younger rediae. In one case a newly freed daughter-redia measured 

 only 0.180 mm. in its most contracted form (fig. 22), while extended 

 it had a length of 0.405 mm. (fig. 23). There is, however, as stated no 

 locomotion, rather only a swaying from side to side, using the posterior 

 protrusions as a pivot. 



Because the variation of these rediae is so great, it might be asked 

 if all belonged to one species. The answer is that without a question 

 they all are the rediae of Echinostoma revolution. The two main 

 proofs are that from every type of rediae the same cercaria emerged 

 having the definite shape, size, number of spines, and arrangement of 

 . excretory system characteristic of E. revolution, and that in a single 

 snail nearly as great variations in each of the above discussed char- 

 acters were found as in the sum total larval stages from all the snails 

 examined of this species. 



