S'ROM THE DOLPHINS OF THE GANGES. 45 



due to the excessive multiplication of particular helminths during 

 certain seasons. As obtains in non-parasitic plagues of all sorts, 

 there is a perpetual rise and fall in their prevalence ; and this is 

 simply due to the presence or absence, as the case may be, of 

 favourable conditions. This is the general explanation of the 

 irregularity observed in the periodical return and degree of viru- 

 lence of the grouse-disease, which I hold to be due to parasites. 

 The same explanation holds good in the case of the disorder I 

 have termed Olulaniasis in cats, also in that of a nemato-helminthi- 

 asis affecting pigeons, due to Ascaris maculosa, also of a similar 

 epizooty occasioned by the four-spined Strongyle (*S^. tetracan- 

 tJius) infesting the horse, also of the well-known parasitic bron- 

 chitis caused by Strongyles in lambs and calves, also of a like 

 disease occasioned by S. commutatus which sometimes carries off 

 hares in great numbers. In short, this explanation applies to 

 many other more or less clearly defined parasitic maladies. 

 Flukes, generally speaking, are comparatively harmless. As a rule, 

 and with some few and notable exceptions (Bilharzia, &c.), only 

 such of them as occupy the liver are capable of doing serious harm 

 to their bearers, whether wild or domesticated. That their pre- 

 sence in the liver of cetaceans is capable of setting up an unhealthy 

 action is abundantly proved by the facts incidentally noticed in 

 this paper. The whole subject of animal parasitism is one of in- 

 creasing interest — zoologists, physiologists, and sanitarians being 

 equally interested in its revelations. This consideration must 

 serve as my excuse for noticing some of the practical points sug- 

 gested by the study of these dolphin-trematodes. 



Appendix. 



Since the completion of this paper I received a letter from Dr. 

 Anderson enclosing " a drawing of a parasite, enlarged 28 times, 

 from the small intestine of Plaianista.'' The drawing had been 

 overlooked, but was fortunately found by Dr. Murie amongst 

 some loose papers. Judging from the illustration (here repro- 

 duced), this is another distinct species of cetacean trematode, and 

 at present new to science. "When I received the specimen Dr. 

 Anderson was making preparations to leave England ; but in 

 reply to my inquiries, he found time to inform me in a second 

 letter that the host was a different individual from that which 

 yielded Distoma campula. He obtained the new parasite in 



LINN. JOURN — ZOOLOGT, VOL. XIII. 5 



