872 



SCIENCE 



[N. S. Vol. XXXVI. No. 938 



multiply more rapidly than paramecia, so that 

 if they are introduced in a culture they soon 

 devour all of their food, after which they ordi- 

 narily encyst. In this state they can be kept 

 indefinitely and when wanted for study all 

 that is necessary is to add, a few days in ad- 

 vance, a vigorous culture of paramecia. After 

 the addition of this culture some didinia 

 usually come out within twenty-four hours, 

 but I have found it necessary in some in- 

 stances to wait several days. They seldom 

 come out in every culture containing cysts, 

 so that it is wise always to keep a number on 

 hand. I have found 50 to 100 c.c. wide- 

 mouthed bottles most satisfactory for this 

 purpose. On several occasions I left culture 

 jars containing cysts together with consider- 

 able debris uncovered during the summer 

 vacation and found in the autumn, at least 

 two months after all of the water had evapo- 

 ated that the cysts came out in about half of 

 the jars. I am, however, of the opinion that 

 the cysts keep better if the liquid is not al- 

 lowed to dry. On one occasion I kept cysts 

 for very nearly a year' in a 5 c.c. bottle full 

 of solution hermetically sealed and found 

 numerous active didinia in less than twenty- 

 four hours after adding a solution containing 

 paramecia. Thus it is evident that after a 

 culture is once established material can be 

 obtained in abundance at any time. 



Didinium is usually described as a barrel- 

 shaped organism. It has two bands of cilia, 

 one near the anterior end, which contains a 

 cone-shaped protuberance with the mouth at 

 the apex, the other near the posterior end 

 where the contractile vacuole is located. The 

 so-called seizing organ, a strand of fibrous tis- 

 sue, extends from the central part of the body 

 to the mouth. The macronucleus is a com- 

 paratively large and conspicuous horseshoe- 

 shaped structure. All of these characteristic 

 features can be clearly seen in specimens 

 killed in Worcester's fluid (a saturated solu- 



^ Since writing this I obtained a good number 

 of active didinia from cysts whicli had been in a 

 5 c.c. vial full of solution two years and four 

 months. 



tion of corrosive sublimate in ten per cent, 

 formalin containing a little acetic acid) and 

 cleared by adding a very small proportion of 

 ten per cent, glycerine and allowing it to 

 concentrate slowly by evaporation. They can 

 also be seen fairly distinctly in living speci- 

 mens which have been kept for a day without 

 food, especially if they are held stationary 

 and slightly flattened under the cover-glass. 

 This can be done readily by slowly removing 

 the water by means of a bit of filter paper. 

 If the cover-glass is slightly tapped after the 

 animals are flattened they burst and then the 

 protoplasmic contents flow out. The ecto- 

 sarc is rather tough and remains intact for 

 some time, resembling a shell. The seizing 

 organ usuallj'^ breaks up and the numerous 

 fibers of which it is composed separate and 

 flow about in the liquid so that they can be 

 cleai'ly seen. The macronucleus rarely breaks 

 and if the cover-glass is lightly touched at 

 ried about in the more liquid cytoplasm and 

 rolls over and over, presenting a view from all 

 sides. In this way the student gets an ex- 

 ceptionally realistic idea of it as a definite 

 structure having a strikingly different con- 

 sistency from that of the cytoplasm. 



The greatest value of Didinium, however, 

 in the study of biology lies in the intense in- 

 terest aroused by the observation of the re- 

 markable phenomenon of feeding. I have re- 

 peatedly seen one of these organisms capture 

 and swallow entire in the course of several 

 seconds a Paramecium ten times its own size. 

 And the essentials in this process can readily 

 be observed by almost any student. 



I should recommend two difl'erent methods 

 in making these observations. 



1. Place a good number of didinia which 

 have been without food for a day in a shallow 

 watch-glass or on a slide and then while 

 observing under the low power, add a drop of 

 solution containing numerous paramecia. Or 

 if more details are desired make on a slide, 

 with a small ridge of vaseline, an enclosure 

 somewhat smaller than a cover-glass. The 

 enclosure should have a small opening on one 

 side. Put a large number of hungry didinia 



