mOM THE DOLPHINS OF THE GANGES. 43 



the A. simplex of Dujardin. On this head I will only add that 

 cetaceans are evidently very much victimized by parasites, both 

 by attacks from within and without ; and it is clear that our know- 

 ledge of the species is exceedingly imperfect. 



On a priori grounds it might be supposed that aberrant ceta- 

 cean hosts would be likely to yield correspondingly aberrant pa- 

 rasitic types. This is not the case, however, either as regards the 

 flukes and nematodes, or, indeed, as regards the cestodes, to any 

 very marked extent. The explanation is not far to seek ; for 

 however divergent particular hosts may be, mere morphological 

 changes in their organs will not of themselves materially alter the 

 conditions of the parasite's existence. To be sure, in the case of 

 the liver-infesting flukes, the mere size of the biliary ducts will 

 tend to modify the size of the parasite ; but it is incapable of 

 altering the type. Thus the common liver-fluke attains very 

 much smaller proportions in the hare and rabbit than it does 

 in the donkey and ox ; but the essential generic character seen 

 in the branched intestine is strictly maintained. Again, the 

 liver-fluke of the giraffe, if it in any degree coordinated with the 

 aberrant characters of its host, would, we might presume, exhibit 

 departures from the type still more marked. The facts, however, 

 show that the only marked differences between the giraffe's 

 Fasciola and the ordinary fluke refer to external configuration. 

 Even the liver-fluke of the elephant (-F. JacTcsoni), the shape 

 of which presents a striking contrast with that seen in the Eumi- 

 nants, still displays the branched intestinal canals, and that, too, 

 in such a manner as to suggest the closest alliance with the Pla- 

 narians. As I have shown in my communication " On the De- 

 struction of Elephants by Parasites " (The Yeterinarian, 1875), 

 the mud-swallowing habits of these huge hosts are eminently 

 favourable to the introduction of fluke-larvse ; and it is worthy of 

 remark that the intermediary bearers likely to be thus swallowed 

 abound in just those very localities where Planarians have their 

 head quarters. 



If my argument is sound, it is clear that neither Platanista and 

 Orcella, on the one hand, nor Inia and DelpMnus, on the other, need 

 be expected to yield fluke-parasites generically differing from those 

 found in Phoccena and other cetacean types. Even wide differ- 

 ences of geographical distribution do not appear to exert any very 

 marked change ; and this is the more remarkable since such varia- 

 tions of habitat by the host might be expected not only to supply 



