1896.] Acid in the Digestion of Certain Rhizopods. 623 
Vorticellidee which take in food particles readily are remark- 
ably free from bacteria in their food vacuoles. Amoeba and 
plasmodia alike exercise to some extent a selective ingestion. 
Greenwood and Saunders claim to have watched Amoeba pro- 
teus for 14 days when surrounded with Bact. termo, vibrios and 
micrococci and the absence of bacteria from the endosare was 
remarkable. They are taken in, it would seem, as unavoid- 
able accompaniments of surrounding food only. Bacteria are 
not recorded to have been observed ingested by protozoa per 
se. 
Another evidence of selective ingestion has been mentioned 
by Dantec, l. ¢., as distinguishing between inert and living 
matter. Active monads or groups of spirilli are placed in 
marked vacuoles of ingestion, containing much of the acid se- 
cretion in comparison to inert matter which is usually invested 
very closely. We therefore have some evidence for assuming 
that plasmodia and Vorticellide distinguish between inert food 
and bacteria. 
(1) Bacteria are rarely ingested except as unavoidable ac- 
companiment of food. (2) Inert food, free from bacteria, is 
invested closely. Septic food within wide vacuoles. (3) In 
sterile environment, food in the stage of storage is the excep- 
tion; in environment of bacteria, storage in acid vacuoles is 
frequent. I have brought these facts before you in this in- 
complete form, because the results are fairly uniform, and with 
the hope of stimulating further observation of the matter. 
These studies require no apparatus outside of the microscope 
and acid indicators. The general suggestion drawn from the 
result has a wider bearing than one would at first sight as- 
sume. For if further study will confirm that the ingestion of 
bacteria constantly prolongs the stage of maximum acidity 
from the usual time of 24 hours to several days in rhizopods. 
The suggestion is that the purpose of the acid is one of (disin- 
fection) killing off bacteria. 
There is a general uniformity of opinion that the presence 
of acid is unaccompanied by any digestive change on nutritive 
matter, which may be stored for many hours before it is dis- 
solved and Greenwood and Saunders intimate that the endo- 
