438 



PARASITES OF ANIMALS 



Thus, for the continuation of the species, there must needs be 

 a contemporaneity of vertebrate and evertebrate hosts. Surely 

 no reasonable person can ascribe this concurrence to merely 

 fortuitous circumstances. In this connection I may remark that 

 Villot, in his account of the migrations of the trematodes, states 

 that the cercarian forms of Distoma leptosomum and D. br achy so- 

 mum of Tringa alpina occur in Scrobicularia and Anthura. These 

 parasites are also found encysted in the gizzard of Tringa. 

 The tapeworms of birds are undoubtedly injurious to their 

 bearers. All the worms appear to be 

 armed with cephalic hooks ; at least, such 

 is the case with the species described 

 by Krabbe, who has supplied figures of 

 the hooks drawn to a scale. Dr Krabbe's 

 beautiful monograph is a perfect model 

 of its kind. In the accompanying figure 

 the hooks have fallen (Fig. 72). On ac- 

 count of the frequency of their occur- 

 rence, some persons have supposed that 

 tapeworms are not injurious to their hosts, 

 forgetting that it is not the mere fact of 

 the existence of tapeworms, but their 

 excessive numbers during particular sea- 

 sons that give rise to avian epizo-6'tics. 

 The same rule holds good with other 

 parasites. Of course, in fledgelings, as 

 also obtains in yearlings amongst our 

 domesticated animals, a very few para- 

 sites are sufficient to prove destructive 

 to the bearer. Thus, as regards the 

 so-called " grouse-disease," during one 

 season it may be due to tapeworms, dur- 

 ing another to strongyles, during a third 

 to excessive abundance of both these 

 parasites. Unfortunately, other avian 

 epizootics, not necessarily due to para- 

 sites of any kind, may be mistaken for 

 helminthic epizooty. The same thing 

 happens amongst quadrupeds. We have, 

 for example, parasitic equine epizootic outbreaks, and likewise 

 non-helminthic equine epidemics (as in the case of the Egyptian 

 horse plague of 1876). The true nature of any epizooty can 



FIG. 72. Head of Tania paradoxa. 

 a, Proboscis retracted ; 6, end of 

 the rostellum expanded. Highly 

 magnified. From an oyster- 

 catcher (Hacmatopus ostraieyus). 

 Original. 



