404 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



phagus hymenoptera deposit their eggs in only one definitely deter- 

 mined kind of insect. Giard asserted with great plausibility, it seems, 

 that each of the species of epicarid isopods definitely parasitises only 

 one species of crustacean, and that two similar parasites found on 

 related species are actually distinct, even when, morphologically, we 

 are unable to discover definite differences of structure or form. The 

 parasitic copepod which Mr. Mesnil and I have just been study- 

 ing 2 under the name Xenocoeloma bntmpti is found on only one 

 species of Poly cirrus (P. ar&nivorus) and not on the related species 

 which are found on the coast, such as P. caliendrum or P. haematodes. 

 What is the mechanism of this definite relationship ? In those cases, 

 as in the insects, where the penetration of the parasite into the host 

 is active, how does the parasite find its host in the immensity of nature 

 and how does it distinguish this host from related species ? Is it by 

 some tropism as J. Loeb supposes, by some olfactory sensation, for 

 instance? In the case of the passive ingestion of spores, as with the 

 gregarine mentioned above, where the two forms of Dodecaceria 

 which live side by side certainly ingest the same sporocysts, why does 

 infection take place in form B only ? And this is related to the other 

 question : How is it that internal parasites, sometimes in the diges- 

 tive tube, sometimes in the deep-seated organs or the body cavity, 

 are able to subsist in their host, possessing immunity to its fluids 

 and phagocytes which digest or destroy the foreign bodies or organ- 

 isms which succeed in finding their way there? We are here faced 

 by the whole problem of immunity. 



How is it that the parasite applies its action to its host, an action 

 which is very definite as regards certain organs ? Thus in numerous 

 instances the parasite, at a distance, through alteration in the meta- 

 bolism of its host destroys the genital glands of the latter, producing 

 parasitic castration. And having produced this result the parasite 

 furthermore induces morphological changes, especially in the sec- 

 ondary sexual characters. It is a remarkable fact that the trans- 

 formations brought about by parasitism sometimes occur in groups 

 such as the arthropods in which experimental castration, no matter 

 how early, has never yet succeeded in producing any change in these 

 sexual characters. And yet Sacculma which infects the crab more 

 or less late in life often profoundly modifies the secondary sexual 

 characters of the crab's abdomen and abdominal appendages. G. 

 Smith has strikingly extended and illustrated the facts first pointed 

 out by A. Giard. 



The list of important questions in general physiology brought up 

 by parasitism could easily be lengthened. 



But let us come back to the morphological problems and to their 

 connection with the more general problems of evolution. And, more- 



2 Bull, biologique de la France et de la Belgique, vol. 53, 1919. 



