NO. 1105. PROCEEDINGS OF THE NATIONAL MUSEUM. 207 



above that the pores of I), salmoni, for instance, may be either unilat- 

 eral or irregularly alternate; the same holds for D. fetragona, and 

 apparently also for Andrya. The characters to be used iu classify- 

 ing the species of any given genus must be determined sejjarately for 

 every genus by an examination of a large series of specimens from dif- 

 ferent localities and from different hosts. The unsatisfactory condition 

 in which I have been obliged to leave the genera Bertia and Andrya 

 is directly attributable to the limited amount of material at my disposal, 

 and the fact that what few, and for the most poorly preserved, specimens 

 I have examined represent only a few localities and a few hosts. 



The influence of a host upon its parasites. — Considerable has been 

 written upon the influence of various parasites ui)on their hosts, but 

 very few authors have ventured to publish upon the influence of the 

 hosts upon the x^arasites. This is ^jrobably due to two reasons: first, 

 to the tendency on the part of many helminthologists to take the host 

 species as a specific character of the parasite, and secondly the fact 

 that the influence exerted by the parasite upon its host is a matter of 

 economic importance, while the influence of the host upon the parasite 

 would be looked upon by most workers as purely of scientific interest. 

 To deny the economic importance of the host influence upon the parasite 

 is, however, to my mind a short-sighted j)olicy, for it must be this very 

 host influence (i. e., environment) which has played an important role in 

 the evolution of species, and which must have resulted in differentiating 

 species and varieties, each of which after a time becomes in some cases 

 at least dependent upon a single host. The genera Demodex, Psoro2)tes, 

 and Chorioptes form excellent examples of the i^oint under considera- 

 tion; in these genera we find varieties which resemble each other so 

 closely that it is often difficult and even impossible to determine the 

 variety without knowing the host; exi)eriments to breed some of the 

 varieties of these genera ui)on other than their regular hosts, even 

 though the experiment animal harbors a very closely allied variety of 

 the same species, have been totally negative. This same variation is 

 noticeable among worms. Specimens of Moniezla expansa taken from 

 Ovis aries are for instance totally different from the specimens of 

 the same species found in Ovls laticauda; the strobihe from this latter 

 host recently studied by Setti, which I have been able to examine 

 through the kindness of my Italian colleague Prof. C Parona, can not 

 I believe be separated specifically from the European form, yet they 

 certainly represent a distinct variety. Lonnberg has called attention 

 to the variation in the hooks of avian cestodes taken from diflerent 

 hosts. I have also repeatedly noticed a variation in the spicules of the 

 same species of nennitode {Strongyl us contortus, for instance) taken from 

 cattle and sheep. To clearly define the varieties peculiar to certain 

 hosts touches directly upon the question of transmission and infection, 

 and on this account I submit that the study is of great economic as 

 well as scientific importance. 



