THE REACTIONS OF DIDINIUM NASUTUM (STEIN) 

 WITH SPECIAL REFERENCE TO THE FEED- 

 ING HABITS AND THE FUNCTION OF 

 TRICHOCYSTS. 1 



S. O. MAST. 



OUTLINE. 



Introduction 91 



Description 92 



Locomotion and General Reaction 92 



Variation in Form 94 



Variation in Size, Fission and Encystment 95 



Feeding Habits 96 



Function of Trichocysts i o } 



Mechanics of Swallowing 108 



Amount and Kind of Food 112 



Choice of Food 113 



Summary 117 



Didinimn nasutum has been known for more than a century. 

 It was described by O. F. Miiller in 1786 under the name of 

 Vortlcella nasitta. In 1859 Stein again described this creature 

 and gave it the now generally accepted name, Didiniuni nasntnin. 

 Balbiani (18/3), however, seems to have been the first to study 

 and record its peculiar feeding habits and his account, although 

 wrong in many respects, had been generally accepted until the 

 paper of Thon (1905) appeared. This paper, however, as the 

 title indicates, deals largely with the structure of the animal and 

 contains only a brief account of the feeding habits. Since this 

 account as well as that of Balbiani seems questionable, I decided 

 to study the reactions of this interesting organism more in detail 

 than had previously been done. 



In order to present the results of this study 2 clearly, it will be 

 necessary to give a brief description of the organism. 



1 From the Laboratory of Experimental Zoology, Johns Hopkins University. 



2 The experiments forming the basis of this paper were performed in the 

 Zoological Laboratory of Johns Hopkins University, in connection with other 

 work, an account of which will be published later. I am much indebted to 

 the university for exceptional facilities put at my command and for financial 

 aid extended to me as Johnston Scholar. 



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