1920] Johnson: Life Cycle of Echinostoma Revolution (Froelich) 361 



somatcriae Levinsen to be built on the "2x4x2" plan, and that of 

 Cercaria pectinata Huet on the "2x7x2" plan. The cercaria of 

 Echinostoma revolution is built on the "2x8x3" plan. Two stylet 

 cercariae excretory systems (unpublished) that I have worked out also 

 show a definite arrangement of the tubules and grouping of the 

 capillaries. 



When many more excretory systems have been worked out I believe 

 that a sound classification of the families, genera, and species can be 

 formulated. However, I am not suggesting that a classification should 

 be built upon this system only. 



ENCYSTED AGAMODISTOMES OR CYSTS 

 The cercariae of Echinostoma revolutum form encysted agamodis- 

 tomes or cysts which are most commonly found in Physa oecidentalis. 

 They vary in diameter from 0.147 to 0.181 mm., but the great majority 

 are about 0.165 mm. Except for the numerous opaque concretions in 

 the tubular divisions of the bladder, the agamodistomes as well as the 

 cyst wall are nearly colorless. These agamodistomes may be found 

 encysted in any part of the body of the snail, from the foot to the 

 extreme end of the viscera, with apparently no preference as to their 

 location. Two hundred and eighty-two snails out of the four hundred 

 examined, or 70.5 per cent, were infected with this stage. Out of the 

 two hundred and eighty-two so infected, ninety-five, or nearly 34 per 

 cent, harbored only the cyst. In this respect this species is different 

 from the majority of digenetic trematodes in that the secondary inter- 

 mediate host is the same as the intermediate host. However, Lebour 

 (1912. p. 442) states that, "in exceptional cases the echinostome cer- 

 cariae may encyst in its first host." Cort (1910, p. 37) finds both 

 the cercariae and cyst of Cercaria trivolvis in the tissues of Planorbis 

 trivolvis. He also found that Cercaria reflexa encysted in the same 

 snail, Lymnaca reflexa (p. 42). Faust (1917) found Cercaria tri- 

 solenata and its cyst in the same host, occurring both in Physa gyrina 

 and Planorbis trivolvis. He also found the cyst of Cercaria biflexa in 

 the tissues of its host beside the cercariae. Prom the above and other 

 similar evidence, I believe that as more echinostome life cycles are 

 partially or completely known that encystment in the same host with 

 the cercariae will be found to be a common family characteristic. 



It often happens that the cercariae escape from the snail only to 

 reenter the same specimen to form the cyst. However, since 34 per 

 cent of the snails containing cysts had only this stage, it is evident 



