io8 LIFE : OUTLINES OF GENERAL BIOLOGY 



bility of certain ants to the successful development of the Large 

 Blue butterfly. 



(6) Allied to the preceding is the linkage established when two 

 animals, widely apart on the scale of being, share a common parasite. 

 Thus the freshwater snail (Limnieus) and the sheep are hosts of 

 the juvenile and adult stages, resjx'Ctively, of the liver-fluke; other 

 freshwater snails (Planorbis, Bulinus, etc.) are hosts of the larva 

 of Bilharzia, which may become a formidable parasite in man; the 

 mosquito carries the malaria-organism, and the Tsetse-fly carries 

 the cause of Sleei)ing Sickness. 



(7) A number of linkages may be groujx'd together as Shelter 

 and Positional associations, where an organism is advantageously 

 sheltered, or lifted, or carried by another. A quaint fish, Fierasfer, 

 shelters inside a Holothurian (sea-cucumlxT), and another Amphi- 

 prion, within a sea-anemone. Many orchids and other plants live 

 as perched epiphytes on trees; many small sponges, zoophytes and 

 worms are carried about on the shells of crabs and other loco- 

 motor marine animals; the sucking-fish or K( mora profits by being 

 transjx)rted by a shark or a turtle to which it may temporarily 

 attach itself; climbing plants use their neighbours as convenient 

 supports. It is not necessary to over-extrt one's ingenuit}^ in finding 

 a utilitarian justification for each and every instance of epiphytic 

 or epizoic association, for many of them are probably casual and 

 unimportant, witness a bunch of ship-barnacles attached to the 

 flattened tail of a sea-snake. There is })robably little significance 

 on either side in the presence of unicellular green Alga^ on the 

 shaggy hairs of the tree-sloth. The attached organisms referred to 

 are simply e.xpressing their constitutional tendency to fix themselves 

 to other organisms or to things, and the attachment is not always 

 appropriate. Yet it must be borne in mind that at some new crisis 

 in the struggle for existence, what was indifferent may become 

 vital, and even of survival value. 



(8) Commensalism is conveniently defineel as a mutually beneficial 

 external partnership between two organisms of different kinds, as 

 in the association of certain hermit-crabs with certain sea-anemones. 



(9) Symbiosis is more intricate - a mutually beneficial internal 

 partnership between two organisms of differtnt kinds. It may be 

 between a plant and a plant, as in lichens; or between a plant and 

 an animal, as in the grwn freshwater sponges, with their unicellular 

 Algjc; or between an animal and an animal, as in the beautiful 

 Infusorians that help digestion in the food-canal of wood-eating 

 white-ants. 



(10) Then there is the large assemblage of parasites, at various 

 grades, such as ectopara.sites and endoparasites, and including 

 almost incredible phases, such as the organic continuity Ix^tween 

 certain pigmy male Angler-fishes, and their more vigorous mates. 



