714 -© The American Naturalist. [August, 
all of the waste products of the skin. Again, if the feathers them- 
selves be examined, another mutualist ( Liothe saculatum) may be seen 
freshening and beautifying their sheen by taking into its stomach all 
dead epithelial cells, etc., with which it comes in contact. Mutualists 
are found everywhere in nature, and wherever found are of essential 
service and benefit to the animals possessing them. From the giant 
cetacean to the microscopic rhizopod, from the savage lion to the timid 
field-mouse, from the kingly eagle to the tiny humming bird, no ani- 
mal is without them. Butler’s epigram: 
* Big fleas have little fleas upon their backs to bite ’em; 
And these fleas have other fleas and so ad infinitum.” 
is mainly true, only I insist that no true mutualist ever bites its host. 
Many mutualists never reside wholly with their hosts, but visit them 
occasionally to render them needful service. The famous crocodile 
bird visits its host in order to pick its teeth; Buphagus, the surgeon ofthe 
buffalo, alights on the back of its host, and, with its sharp, lance-like 
beak opens the cells of encysted larve and removes them; the Euro- 
pean starling performs a like service in a " wolves ” from the 
backs of cattle. 
In matters of the toilette many animals are entirely dép demie on 
the ministrations of mutualists. This is notably the case with many 
of the fish family. I placed two gilt catfish, whose skins had been 
thoroughly cleaned with a solution of salt water and borax, in a tank 
of filtered water in which there were no eltes, the mutualists of 
this species of fish. In two days their skins had lost their beautiful 
golden sheen and had become dull and lusterless. "The fish themselves 
clearly showed by their actions that they were not in good health. 
They remained at the bottom of the tank almost without motion. I 
then took them out and found that their skins were covered with a 
slimy mucous exudate. I placed them for a few moments in a tank 
of pond-water in which there were multitudes of Gyropeltes. After al- 
lowing them to remain in this tank for a few moments, they were re- 
moved and examined, and thousands of these mutualists were discov- 
ered greedily devouring the mucous. After a day's residence in the 
pond water their skins had recovered all their lustre and beauty, and 
the fish showed by their aetions that they had regained their health. 
A truly remarkable mutualist is found associated with the crayfish. 
It belongs to the genus Histriobdella, and its office is analagous to that 
of the vulture, the jackal, and the burying beetle which remove car- 
rion. It is exeeedingly agile and is altogether one of the most unique 
in appearance of all animals. It may be described as a two-legged 
LUE ee ARR A o Eee s ghe Se ES T RR Svo Mey a 
Moy reapse CS edet: 
MOT CARA Su IURI T aer Le ERR 
