1894.] Psychology. 713 
PSYCHOLOGY. 
Mutualists.—Many animals which are found associated with 
-other animals and which are usually termed parasites are, in fact, true 
mutualists. I mean by the term, mutualist, an animal which gives a 
quid pro quo or specific beneficial service to the host which affords it 
sustenance and domicile. A true parasite feeds on the food or the 
physical juices and structures of its host without rendering auy recip- 
rocal service whatever. Thus, the family Pediculide (P. corporis, P. 
capitis, etc.), found associated with man, are true parasites, while the 
family Ricinie, found associated with birds, are true mutualists. I am 
fully aware of the fact that I antagonize the opinions of entomologists 
(who regard all these little creatures as parasites which are to be de- 
stroyed as soon as discovered, inasmuch as they consider them detri- 
mental to the health of the animals upon which they are found), for I 
consider most of them absolutely necessary to the health and well- 
being of their hosts, and their absence to be an indication of disease in 
some form or other in those animals on whose bodies they are not to 
be found. Careful observation has taught me that these faithful little 
hygeinic servitors immediately abandon the bodies of fowls which are 
the victims of cholera and kindred diseases. Porcine mutualists be- 
have in a like manner when their hosts become diseased. I had 
thought with others until recently, that these corporal scavengers and 
toilette assistants were parasites, but systematic and painstaking obser- 
vation has taught me otherwise. In the first place, microscopic exam- 
ination shows that these creatures have no suction apparatus like fleas 
(Pulex) and lice (Pediculus) for the purpose of sucking up the blood 
and juices of their hosts. Their jaws are usually armed with a simple 
pair of incurvated scrapers with which they scrape the surface of their 
hosts’ bodies. Their stomachs never contain the blood of their hosts, 
but are always filled with exfoliated epithelium and kindred super- 
ficial debris. Supported by these observations alone, the fact at once 
becomes evident that these creatures are not true parasites; but there 
is yet more testimony to be adduced in favor of these hitherto maligned 
-coadjutors and promotors of animal hygiene. If one carefully separ- 
ates the feathers on the body of a fowl and uses a good lens (10 diam.) 
hé may observe Liothe pallidum, a true mutualist, busily engaged in 
removing exfoliated epithelium (scarf-skin) from the body of its host. 
It thoroughly cleans its allotted area, scraping away and swallowing 
