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scale of vertebrates. On one occasion when walking in 

 the Zoological Gardens, near the elephant house, an unduly 

 distended mouse was labouring to cross the path in front 

 of me. Having easily killed it, I thought to obtain some 

 embryo mice ; albeit in this I was disappointed, or, rather, 

 perhaps, gratified, inasmuch as the dissection soon revealed 

 the fact that the distension was entirely owing to the 

 presence of several large filariae within the stomach. 

 (Spiroptera obtusa, Rud.) Clearly these parasites had 

 seriously incommoded the little rodent's progress, and, as 

 in the case of the minnow, had led to its capture and 

 death. 



But I now proceed to direct your attention to far more 

 cogent evidence, the value of which will be better under- 

 stood if I first explain the special character of the 

 tapeworms that are so injurious to fresh-water fish. The 

 tapeworm of the minnow just alluded to was an immature 

 ligule. All the ordinary ligules of our fresh-water fish 

 are temporary residents awaiting their passive transference 

 to the body of some higher, definite, or ultimate host. 

 This final host is usually a water bird, which, preying 

 upon the fish, swallows the parasite, and after a very short 

 space of time the parasite itself arrives at sexual maturity. 

 As long as the ligules remain in the fish their development 

 is not complete, but when once subjected to new and 

 suitable conditions their arrival at maturity is only a 

 question of time. If the new residence is unsuitable to 

 the species of parasite, no further development will take 

 place ; in other words, all parasites requiring a change of 

 hosts must have an environment specially adapted to their 

 individual wants. 



With the structural changes thus brought about we are 

 not now concerned, but it is worth while remarking that 



