PARASITISM AND SYMBIOSIS CATJLLERY. 405 



over, in reality they are not independent of the physiological prob- 

 lems. 



The reaction of parasitism on morphology is striking. It is even 

 true that there is no category of analogous facts in biology which 

 shows the same breadth. Parasites are distinguished from other 

 forms belonging to the same groups by such marked features that 

 the affinities of these parasites often become unrecognizable. In the 

 various groups the transformations which are undergone present an 

 evident parallelism. 



Parasitic degradation is a well-known idea. The more profound 

 the parasitism — that is, the more the parasite uses the activities of its 

 host's organism to insure its own nutrition — the more it is deformed 

 and simplified; in a general way the organs of locomotion and of 

 the senses retrogress and finally disappear. It even goes so far that 

 no trace of the primitive nervous system can be found. The organs 

 of digestion become simplified when the parasite, absorbing substances 

 already completely elaborated and assimilable, is dispensed with the 

 labor of first bringing them itself to this condition ; hypertrophy then 

 takes place only in an organ of storage such as the " liver " or hepato- 

 pancreas. The digestive tube on the other hand, sometimes disap- 

 pears completely, as in the cestodes, and the animal, bathed in assim- 

 ilable nourishment, feeds by simple osmosis through its integument ; 

 the reproductive organs especially are changed, and in a general 

 manner hypertrophied, but in different ways. Sometimes parasitism 

 obviously brings on hermaphroditism. Sometimes it exaggerates 

 sexual dimorphism, making of the female, stuffed with eggs, a giant 

 on which the dwarf male lives permanently like a parasite. In the 

 one case as in the other the number of eggs produced by parasites is 

 enormous : there is in this enormous production of eggs a compensa- 

 tion for the extreme mortality of the embryos and larvae which do 

 not reach in time the indispensable host ; an essentially adaptive com- 

 pensation which is likely to give rise to teleological illusions. Thus 

 parasites finally become nothing more than voluminous egg sacs, and 

 the animal's cycle after the host is found resolves itself into building 

 up the substance of these eggs, laying them, finally incubating them 

 until the hatching of the larvae. 



Thus there is a great simplification of the various systems which 

 do not take part in reproduction — a degradation if you wish, in rela- 

 tion to free forms ; but that is a subjective point of view, and we can 

 just as well say that parasites show a specialization carried more or 

 less far, and, all in all, an adaptation which often reaches a very great 

 perfection. In their way parasites are much specialized forms which 

 may be regarded as in a sense extremely high in organization. We 

 shall illustrate these ideas by a few examples and we shall see in par- 

 ticular how a single group often presents successive stages of trans- 



