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 Hving 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 have a hydroid zoophyte growing 

 upon it with Vorticella 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, and so on. In each case it is 

 impossible to decide how far commensaHstic 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 Symbiosis. Numerous instances of sym- 

 biosis occur. One of the best examples is that of radiolarians 

 and their partners the unicellular algae, 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 Endoparasitism. 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 oi like individuals is termed a colofiy, which is 

 very common in Protozoa^ Porifera and Coelenterata^ the 

 lowest phyla. 



