ae ae 
ay 
THE SOCIAL LIFE OF THE LOWER ANIMALS. 525 
But among these independent existences there are a certain num- 
ber, who, without being parasites, cannot live without aid, and 
who claim from their relations, sometimes a simple resting place 
at the same table to divide the meal of the day ; we daily discover 
some which pass as parasites, but which, however, do not live in 
any other way at the expense of their host. 
Although a copepod crustacean is installed in the body of an 
Ascidian, and intercepts the passage to its mouth of some good 
tidbits, yet we cannot regard it as a parasite. 
But should an animal kindly render a service to his neighbor, 
whether keeping his set of teeth in order,* or in removing the 
detritus which encumbers certain organs,f we cannot say that it 
isa parasite. It is no more a parasite than he who squats by the 
side of a vigilant and clever neighbor and quietly takes his siesta,} 
or he who contents himself with the fragments which fall from the 
jaws of his acolyte.§ It is no more a parasite than he who, like 
the Remora, lazily anchors himself to a good swimmer, and fishes 
by his side without fatigue to his fins. All these animals are no 
more parasites than the traveller who installs himself in a pleas- 
ure carriage, extends bis hand in passing, or carries a crust of 
bread in his pockets. There are also mutual services rendered 
among several species, services performed by reciprocal kindness, 
and mutualism can even take its place by the side of commensalism. 
Those which deserve the name of parasites are maintained at the 
expense of their neighbor, whether they reside voluntarily in its 
i or abandon it for a while after each repast, as the leech or 
ea. 
The true parasites are very numerous in nature, and we should 
wrongly imagine that they live an unhappy and monotonous life. 
There are among them some alert and vigilant enough to sustain 
themselves for a part of their life, and only need aid at determi- 
nate periods. They are not, as has been believed, exceptional 
and strange beings without any other organs than those simply for 
P S a a 
* A plover enters the mouth of th Jile and the dél 1 
from its immobile tongue, cannot get rid of. It is a living toothpick. This fact was 
already known to Aristotle and has been since verified. 
t The opalinas of the rectum of frogs. 
$ A screech owl in Mexico places itself under the care of a small subterranean ro- 
dent, excessively alert and vigilant, the spermophile. He acts as sentinel at the door 
of this house, say the people of the country, and the owl lives in perfect quietude. 
$ An annelid of the genus Nereis establishes itself by the side of and in the same 
thell with the hermit crab. 
*. shat the animal 
