THE WEB OF LIFE 



295 



partner an effective protection against the appetite of 

 cuttlefishes. It is a fine case of diamond cut diamond, 

 the thrust and parry between crab and cuttle. 



A number of animals which clean up others without utiliz- 

 ing any living material should be 

 ranked with the commensals, not with 

 the parasites. This is true of many of 

 the so-called fish-lice (Argulidse), which 

 are scavengers of the skin of carp and 

 other fishes, and of various insects and 

 mites (e.g. Trichodectes, Philopterus), 

 which do the same for mammals and 

 birds. Another example is the plover, 

 which Herodotus accurately described 

 as cleaning the mouth of the crocodile, 

 removing leeches and other parasites 

 from the huge gape. 



Symbiosis. It seems to us justi- 

 fiable and useful to restrict this term 

 to the mutually beneficial internal 

 partnership of two organisms of differ- 

 ent kinds. 



In most Eadiolarians pelagic Pro- 

 tozoa usually with siliceous skeletons 

 there are symbiotic Algae which 

 used to be known as ' yellow cells '. 

 They are unicellular plants embedded 

 in the transparent living matter of 

 the Radiolarians, and a very profitable 

 partnership has been established. 

 Being possessed of chlorophyll, the Algse 

 can utilize the carbonic acid formed 



FIG. 49. A colonial 

 Radiolarian, Col- 

 lozoum inerme. 

 (After Brandt.) 

 The small spheres 

 are the units 

 composing the 

 colony ; each is 

 accompanied by 

 partner Algae ; all 

 are imbedded in 

 a transparent 

 matrix. Greatly 

 enlarged. 



