Physiological Studies on Certain Protozoan Parasites. 155 



time of disappearance differs in different experiments the relative order for 

 the four forms seems to be quite constant. 



It will be seen, therefore, that in respect to their ability to live in the 

 body of the host after the latter's death, just as in many other respects, 

 they show certain characteristic physiological differences. The relative 

 length of life of the four forms is roughly 1.5 : 2 : 2 : i. Just what factors 

 cause the death of the parasites under these conditions is not certain, but 

 probably the products of decomposition present play an important part. 

 As will be shown later, some of these substances are markedly toxic, though 

 each of the four forms is affected differently by them and the immediate 

 cause of death may perhaps be different in the different cases. 



EFFECTS OF CO2 ON THE PARASITES. 



Carbon dioxide certainly occurs normally in the intestine of Diadema and 

 therefore small quantities of it can not be markedly injurious to any of the 

 four forms in question. With large quantities the case is different, all being 

 killed in a fairly short time, though the four forms are by no means alike in 

 their resistance. If a drop of fluid containing them be exposed to a contin- 

 uous stream of the gas in a gas-chamber and kept under constant obser- 

 vation, it will be seen that D, the least resistant form, is rendered motionless 

 in 3 or 4 minutes; ^ comes next, the time required for it being 5 to 6 minutes; 

 C remains active for about 15 minutes, and B for from 45 to 60 minutes. 

 The ratio of resistance is therefore roughly 1.5 : 15 : 4 : i; 5 being the 

 most resistant form and D the least. The same order of resistance was 

 found to hold if instead of subjecting the animals to a stream of gas they 

 were placed in a relatively large quantity of sea-water charged with CO2 

 by means of a "sparklet" bottle. 



EFFECT OF H2S ON THE PARASITES. 



Hydrogen sulphide begins to appear in the intestine of Diadema in con- 

 siderable quantities soon after death. The vapors given off on gently 

 warming a little of the intestinal contents taken 12 hours after death darken 

 lead acetate paper very perceptibly, and after 24 hours the reaction may 

 often be obtained without the application of heat at all. As this substance 

 might be suspected of playing a part in causing the disappearance of the 

 parasites from the intestine of the host after death, it was of interest to 

 examine its effects on the four forms in question. As might be expected, 

 it proved to be markedly toxic to all, but the different forms resisted it 

 in pronouncedly different manner, and, furthermore, the order of resistance 

 is different from that shown to CO2. C, for example, which is next to the 

 most resistant form to CO2, is killed first when a current of the gas is passed 

 through the gas-chamber containing the animals, the time required being 

 only I to 2 minutes; > comes next, with a period of about 3 minutes; A lives 

 3 to 4 minutes, and 5 10 to 15. The ratio of the resistance of the four forms 

 is therefore approximately 2.5 : 8 : i : 2. 



