304 Illinois State Laboratory of Natural History. 



Several methods were followed in the preservation of the 

 tows. Some of the material was killed in 50% alcohol and 

 then transferred to 70% alcohol; another part was killed in 

 95% alcohol ; and picro-nitric acid, followed hy 70% alcohol, 

 was extensively used. Flemming's fluid as employed by 

 Rousselet ('93), and a solution of potassium permanganate 

 recommended by Zacharias ('94, p. 88) were also used. A 

 second paper by Rousselet ('95), in which he describes the 

 method of killing with | % osmic acid and then preserving 

 in a 2.5% solution of formalin, was received in time to be 

 used in connection with the work in 1895. In 1896 nearly 

 all the qualitative catches were killed and preserved in 2% 

 formalin. Good results were obtained in almost all cases. If 

 one is limited in time and can have but one killing agent, 

 formalin comes nearest being the ideal all-around killing fluid. 

 By far the best results, however, are obtained with osmic acid, 

 according to the process worked out by Rousselet ; but the 

 use of this agent requires much time and patience. 



PROTOZOA. 



The Protozoa present a very attractive field for study, 

 including a large number and great variety of species. We 

 find here a more marked difference in structure and form 

 than among the Rotifera; yet all the Protozoa are either 

 simple one-celled animals or colonies of single-celled individ- 

 uals. 



They exhibit a great variety of structural detail, and range 

 in length from 6 mm. to ,.005 mm. Many of the forms are 

 marine, but a great number occur in fresh or stagnant 

 water. Notwithstanding their diversity of structure, the 

 Protozoa have, as a rule, the protoplasm or body substance 

 differentiated into an inner part, called the endoplasm, and 

 an outer envelope, called the ectoplasm. This differentiation 

 may be temporary, as in the Rliizopoda, or permanent, as 

 in the Flagellata and Infusoria. A single nucleus is usually 

 present, though some species have more than one. The 

 Infusoria are characterized by the presence of a paranucleus, 

 or micronucleus, in addition to the larger nucleus, or mega- 



