70 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



several other things at this time. For one thing, the lists given here 

 are such as could be compiled in the time at the writer's disposal and 

 while reasonably comprehensive must necessarily be incomplete. For 

 another thing, our total knowledge in regard to the life histories of 

 heteroxenous helminths is very slight. As already stated, we know the 

 life histories of approximately i per cent of the known tapeworms, 

 and this status is sufficiently representative of conditions for all heter- 

 oxenous worm groups to need no detailed statement in regard to 

 the other groups. There may be important intermediate host groups 

 of which no member has yet been incriminated. We know about 143 

 life histories involving arthropods ; there are certainly hundreds, per- 

 haps thousands, of such life histories still to be ascertained. 



Admitting all of these defects in our data, we are nevertheless 

 justified in saying that the lists presented here will be of value in 

 affording the student a clue as to the probabilities in beginning a 

 search for the intermediate host of a heteroxenous worm parasite, or 

 in considering the probable identity of a larval worm found in an 

 arthropod. This will fulfill one of the purposes of this paper — to aid 

 the student. The young students of to-day will include among their 

 ranks the competent scientists of to-morrow. 



Another purpose of this paper is to point out the opportunities for 

 cooperation among scientists in adding to our knowledge of the life 

 histories of parasitic worms. Zwaluwenberg, an entomologist, has said 

 recently : " The interrelationships of insects and nematodes is a 

 subject of which most entomologists seem to have little adequate 

 conception." Some months ago, in discussing the scope of this paper 

 with Dr. L. O. Howard, the writer told him that he expected to call 

 attention to the fact that our knowledge of these life histories had 

 come almost entirely from the parasitologists, and that the workers 

 on insects and crustaceans had aided very little in the process. Dr. 

 Howard, characteristically, suggested that this be done very diplo- 

 matically. It is the writer's intention to do this diplomatically. It is 

 primarily the business of the parasitologist to ascertain the life his- 

 tories of the parasites with which he deals. It would not be in order 

 to ask the specialist on insects or crustaceans to ascertain the life 

 histories of the larval worms which he finds in these insects and 

 crustaceans, nor would it add greatly to our knowledge to have per- 

 sons unfamiliar with parasitic worms publish findings in an unfamiliar 

 field. 



Nevertheless, there is an opportunity for cooperation between the 

 workers on parasitic helminths and the workers on their arthropod 

 hosts, and little advantage has been taken of this fact in the past. 



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