73 



1809, 1816, 1824, 1830. 1853, and 1860. So much we learn from 

 Dr. Cobbold's work. 



I should be anxious to know if 1866 and 1865 had been char- 

 acterised by a like outbreak. 



In conclusion, I would observe that it is not certain that the 

 infusoria I have found are identical with those I have referred 

 to as described by Steenstrup ; or is it certain that those he 

 described were identical with those producing the liver-fluke in 

 sheep ; all that we can at present affirm is that they are more or 

 less related to them. Indeed, on carefully reading Dr. Cobbold's 

 work and the important one of Steenstrup, I am struck with the 

 great uncertainty which exists on this subject. The distoma 

 which Cobbold calls fascicula hepaticum has been most care- 

 fully figured and described, and Dr. Cobbold has figured and 

 described other forms. The ova were figured by Lenckart, 

 and in page 168 of Cobbold's work is figured a ciliated free 

 swimming embryo of fasciola hepatica from Lenckart. But the 

 former writes : — " Up to this time at which I write, the subsequent 

 changes which the embryo of fasciola hepatica undergoes are not 

 clearly defined. Mozlinie and others have referred to certain of the 

 cercaricB and the higher forms of tremadode latvcE as the young of 

 our fluke, but it cannot yet be positively asserted that we know the 

 higher larval conditions of this parasite." 



How far can we be sure that these larva forms are always 

 developed ? Are they the only forms in which the true fluke is 

 ultimately produced .? Do we know that the ova of the fluke, if 

 swallowed by the sheep, will produce other flukes without under- 

 going a similar round of nurses and their off'spring ? Are the 

 fresh-water mollusks the only animals that form the host of 

 these infusoria.? All these questions must be fully answered 

 before we can be said to have arrived at a satisfactory know- 

 ledge of their history. 



Again, the liver-fluke in sheep appears to have been prevalent 

 at diff"erent seasons ; but if they merely introduced the larvae 

 into their systems through the water they drank, there is no 

 reason in particular why some seasons must have so prejudicially 

 affected them, as every season there must have been sufficient in 

 the water to effect this purpose ; unless, indeed, the particular 

 form in which they can be introduced into the animal is that 

 of the cercarian, and that these are only produced at certain 

 intervals. I have stated that in 1866 I met with this form in 

 great abundance, and not again till 1880. 



APR ^"87 



