PHYSIOLOGICAL STUDIES ON CERTAIN PROTOZOAN PARASITES OF 



DIADEMA SETOSUM. 



By Merkel H. Jacobs. 



INTRODUCTION. 



It has been shown by the author and others that different species of 

 protozoa have certain physiological characteristics, often almost as striking 

 as their morphological ones, and which are probably of considerable signifi- 

 cance in the interpretation of their habits of life and their relation to their 

 environment. The study of such characters in the past has been greatly 

 neglected on account of the general belief of biologists that they are exceed- 

 ingly inconstant and subject to modification by external factors and there- 

 fore of little value in an understanding of the fundamental nature of the 

 organisms possessing them. Previous attempts to detect characteristic 

 physiological differences between different forms have for the most part 

 been confined to organisms whose general habits of life are so dissimilar as 

 to leave room for the objection that the differences observed may have been 

 due merely to the effect on the animals of the different environments to 

 which they have been accustomed and not to any innate peculiarities of the 

 organisms themselves. 



It occurred to the author as desirable to test, if possible, a series of forms 

 which naturally live under essentially the same environmental conditions, 

 and which may be assumed to have done so for many past generations, in 

 order to see whether they show greater likenesses than a number of forms 

 selected at random, or whether each has preserved its individuality in spite 

 of the similarity of its environment. In the case of a number of parasitic 

 protozoa living together in the same organ of the same host, we have a 

 favorable opportunity for such a study. Even here we can not assume 

 that each form has exactly the same environment, but, there can be no 

 question that this environment is far more similar than that of a number of 

 selected free-living forms. Furthermore, in order to become parasites of the 

 same host the nature of the forms in question must have been more or less 

 changed. In the case of intestinal parasites, for example, it would be 

 necessary for them to acquire a resistance to the digestive juices of the 

 host and to become accustomed to a more or less anaerobic habit of life, etc. 

 Such changes would necessarily be in the same direction for all of them. 

 If, therefore, a number of parasitic forms, which have had to meet and adapt 



149 



