136 LIFE : OUTLINES OF GENERAL BIOLOGY 



adequate. Dr. L. R. Cleveland has made a careful study of certain 

 beautiful infusorians that have their exclusive home in the food- 

 canal of many kinds of wood-eating Termites. The infusorians find 

 shelter and fodder, and the Termites or White Ants cannot thrive 

 on their dry food unless their partners (should we call them "sym- 

 bions", "symbionts", or "symbiotes"?) are present. But symbiosis 

 it is, and we now see that it may be between plant and plant, between 

 plant and animal, and also between animal and animal. 



PARASITISM 



So far we have discussed the mutually beneficial linkages between 

 two different kinds of organisms, and distinguished external 

 commen.salism from internal symbiosis. But parasitism is a one- 

 sided nutritive relation, which is more or less injurious, yet not 

 usually fatal to the host. It is a relation which relieves the parasite 

 from most of the activity or struggle that is usually involved in 

 procuring food; and thus it tends to favour or induce some degree 

 of simplification or degeneracy. The parasitism may be (a) between 

 one animal and another, e.g. between tajx^worm and dog; or (b) 

 between an animal as host and a plant as parasite, e.g. the salmon 

 infested with a Saprolegnia fungus; or (c) between one plant and 

 another, e.g. dodder on clover; or (d) between a plant as host and 

 an animal as parasite, e.g. the ears of wheat infested by a minute 

 threadworm {Tylenchus tritici), which causes "ear-cockles", {e) In 

 rare ca.ses, e.g. Boncllia, the male is a parasite of the female. 



VARIETY OF PARASITIC RELATIONS.— Parasitism takes 

 many forms and occurs in many degrees, so that hard-and-fast 

 definition is imj^ossible. An organism may be parasitic during one 

 period of life and independent at other times; thus the larval stages 

 of the hookworm live in the soil, the adult stage is reached in 

 man's intestine; or again the young forms of the horsehair worms 

 (Gordius) occur in insects, whence the adults emerge into the water. 

 The parasitism of the strange Copeix)d Crustacean {Lcrncea hran- 

 chialis), d'omnion on the gills of the haddock, is confined to the 

 female, and does not begin until after pairing has taken place. 

 In two or three of the Angler-fishes of the Lophius tribe, which 

 inhabit the mid-water zone, intermediate between surface waters 

 and abyssal, the male is actually an external parasite of the female. 

 The parasite may be externally a.s.sociated with the host, like 

 the mange-mites on dogs; but this cctopara.sitism has also its grades, 

 varying with the extent to which the host is punctured or penetrated 

 by the parasite. Thus the very degenerate parasite Crustacean 

 Sacculina protrudes visibly on the ventral surface of the parasitised 



