74 MUTUAL RELATIONSHIP OF ANIMALS. 
Animals of a similar structure or species often find 
it advantageous to seek for food together, either for mutual 
protection (herbivora) or for mutual support in attack 
(carnivora). These are said to be gregarious. 
In some cases, animals of a different kind are found in 
partnership. Strange combinations of two or more animals 
of divergent structure are found dwelling together. If 
this partnership appears to be an equal one, with mutual 
benefit accruing, it is termed Commensalism. A good 
instance is found in the common hermit-crab, which has a 
particular species of sea-anemone living upon its shell. If 
one organism obtains all the benefit, then commensalism 
shades off into Ectoparasitism: In many instances it is 
impossible to decide between the two categories. 
Sedentary marine organisms are nearly always intimately 
connected. A cockle may havea hydroid zoophyte growing 
upon it with Vorticel/a upon the hydroid zoophyte and a few 
tunicates with small Folyzoa upon them. A tubicolous 
worm may be fastened to the back of an oyster, with acorn- 
barnacles covering its tube, andso on. In each case it is 
impossible to decide how far commensalistic or ectoparasitic 
proclivities predominate. 
Ectoparasitism also gradates into the carnivorous habit. The lion 
can hardly be termed an ectoparasite on the antelope, but the hagfish 
has often had this appellation because it feeds on fish, and a leech is 
another difficult instance. 
If commensalism becomes still more intimate, and the 
two organisms become inseparable in their vital processes, 
the union is termed Syzdzoszes. Numerous instances of sym- 
biosis occur. One of the best examples is that of radiolarians 
and their partners the unicellular alge, termed yellow-cells. 
The plant furnishes the oxygen required by the animal, and 
itself uses the carbonic acid produced by the animal. 
As in the case of the physical connection, so in this 
organic union the partnership may be one-sided, in which 
case it is termed Lxdoparasitism. In endoparasitism the 
parasite depends for nutrition upon its host, living more or 
less permanently in its body. Lastly, we can see that the 
organic union of Ze individuals is termed a colony. which is 
very common in Protozoa, Porifera and Cwlenterata, the 
lowest phyla. 
