INTRODUCTION. VII 



but since they are seldom looked for, they are often 

 passed by unnoticed. For their study, however, syste- 

 matic collections should be made, and for this purpose 

 the superficial ooze at the bottom of still-water should be 

 examined, after it has been allowed to settle for some 

 time in a suitable dish. They are common in the slime 

 of submerged rocks, stems and leaves, and especially 

 so in moist sphagnum, and are to be found almost 

 everywhere in moist situations not too much shaded, 

 among decaying logs, mosses, lichens, and on the bark 

 of trees. 



In Professor Leidy's book, references to the literature 

 of every species are given, with the names under which 

 they have been described. In this book these names have 

 been given as synonymes, with the names of the authors 

 who have described them without the references. Those 

 who possess the older works on the Infusoria, will 

 doubtless find these synonymes of some assistance. The 

 habitats given in the texts, are only intended to indicate 

 where the species are likely to be found ; and too much 

 importance should not be attached to them in naming 

 specimens. 



The index is arranged to serve as a check-list, so that 

 the student can readily keep a record of the species that 

 he may find. 



Measurements are all given in micro-millimetres ; or, as 

 more conveniently named, in micras. A micra is the 



