T9 NINETEENTH REPORT. 



new Arcella is closely united to the ectoplasm of the maternal sarcode. 

 Soon the shell begins to take on its characteristic shape, and fission of 

 the protoplasm results in two daughter cells, each with typical amoeboid 

 activities. The young cell is distinguished by a lighter color, which 

 gradually deepens with age. 



Chemical and Physical Characteristics. As has been anticipated, the 

 shell of Arcella is chitinoid in composition, resembling the protective 

 exoskeleton of the Arthropods. It is insoluble in all ordinary acids and 

 alkalies, a feature of immense significance to the organism. In some ex- 

 periments of my own, digestion in water at room temperature for half a 

 year failed to produce any appreciable effect, while it reduced insect 

 structures entirely, with the exception of the elytra and mandibles. 



The brownish color is evidently due to iron salts deposited in the shell 

 substance. Awerinzew's "Arbeit nach enthalten die Arcellashalen Eisen," 

 as well as Khainsky's statements, support this view. Further support 

 is derived from the fact that the animals thrive best in water rich in 

 minerals, and do not propagate in the acid water of bogs where Actino- 

 phrys is found. As to the general chemistry of the shell, Khainsky sug- 

 gests that in the young shell an oxyaminoacid is present, which is altered 

 with age by the substitution of iron. 



REPRODUCTION. 



The reproductive processes in Arcella are remarkably varied and com- 

 plex for an animal of so simple organization and especially one of free- 

 living habit. All the elementary types of reproduction know^n in the 

 Protozoa have been described for this single genus. 



Fission. The simplest manner of reproduction is that in which a 

 mature animal divides into two daughter cells, and it is this type of propa- 

 gation that was first described in the genus. As physiological maturity is 

 reached, the protoplasm of tlie maternal cell proceeds to the mouth of 

 the shell, where, by the absorption of water, it swells to a size equal to 

 or greater than the shell. A new shell membrane is formed which takes 

 the characteristic shape immediately. This gives the phenomenon of 

 two shells applied face to face, with the sarcode mass within. While 

 further development of the shell proceeds, there occurs a thorough 

 manipulation and reorganization of the nuclear material. The two nuclei 

 finally divide simultaneously, and at the end of two to four hours from 

 the start of the process, equal fission occurs, resulting in two individuals, 

 which after a period of inactivity, resume the ordinary life activities. 

 This is one of the more common reproductive processes, and occurs with 

 some frequence under all living conditions, but, as with other types, is 

 subject to epidemics under suitable environmental states. 



