No. 624] SHOBTER ARTICLES AND DISCUSSION 



system lies in the fact that it can be studied entirely in the living 

 cercariae. The writer has used this method with profit, but in 

 addition has worked out a method of staining the genital organs 

 in the preserved larva?. This method can be utilized when the 

 worker has access only to preserved larvae. While the excretory 

 system is indeed a conservative system, the genital system is 

 probably more conservative and less likely to change from cer- 

 carial to adult stage. It has been found to be remarkably simi- 

 lar in the large, yet variable in minor, details in groups of cer- 

 cariae known to be related through other organs or systems. 

 The best description of a cercaria is probably that which includes 

 both the excretory system as worked out in the living animal and 

 the genital cell masses as depicted in carefully preserved and 

 stained material. 



A mere superficial description of the worm is a distinct burden 

 on the literature. The cercaria should be carefully studied in 

 minute detail or not at all. It is the nicety of technic and care 

 in observation which have yielded the number of species now 

 known and bids fair to increase the number vastly in the next 

 few years. It is necessary, then, to urge the investigator in this 

 group to use the utmost care in his work, to describe the minute 

 parts of the organs, and to record the complete biological data 

 available that these records may be of use in life-history inves- 

 tigations. 



In order to place the more important biological data of de- 

 scribed cercariae in the United States in a convenient form, a 

 table has been prepared to cover the groups, the authors and 

 dates of the naming of the species, the hosts, localities and dates 

 of collections and the per cent, of infection (see Table I). The 

 same data have been collated from the standpoint of the host 

 in Table II. 



A study of the described species shows that the great bulk are 

 distome larvae. Most of these fall into three groups, the stylet, 

 echinostome and forked-tailed cercariaB. The former group bear 

 evidence of relationship to the Plagiorchiidae ; the echinostome 

 cercariffi are known to be larval Echinostomidae, and the forked- 

 tailed cercariflB are probably larval schistosomes. The life his- 

 tory of only one species in the group has been worked out with 

 certainty, that of Cercaria Zmorc/n'.v f(iiri>orti\ with I'latmrbis 

 trivolvis as larval host, a chironomid lai-va as iiitrrnuMiiate host, 

 and Ictiobus spp. as definitive hosts. Of the species recorded 



