642 ON THE DEVELOPMENT OF BlLHARZIA HiEMATOBIA, 
of only -gVo o^, up to the comparatively large magnitude of 
Woo" lengthways. 
According to the evidences thus placed before us, in respect 
of the occasionally eccentric disposition of the spine, and 
considering also the possibility of the occurrence of more 
extreme departures than those which I have myself observed, 
I think that Dr. John Harley will have to admit that there 
is no good ground for asserting the probability of any 
specific differentiation between the adult parasites coming 
from the Cape and Egypt respectively. And here I may 
remark that, in determining delicate questions of this kind, 
the lessons gained by a prolonged study over the whole 
field of parasitology materially aid one iin arriving at the 
truth. 
Taking a more extended view of the significance of these 
singular chorional outgrowths, I think it is possible to deduce 
conclusions of higher import than the particular one at which 
Dr. Harley appears to have arrived. I am not aware, indeed, 
that the ideas which I have for some time entertained on the 
subject have occurred to any other student of helminthology ; 
but in these minute and frequently inconspicuous spines I 
clearly recognise the early efforts of Nature, so to speak, to 
form or evolve a special organ, which, in the eggs of certain 
other parasites, becomes capable of attaining a relatively 
prodigious degree of development. To me it seems that the 
little process in question is a kind of rudimentary holdfast; 
and, as such, it may be reckoned as the homologue of a 
variety of egg-appendages. 
Eleven years ago, Mr. Edwin Canton discovered some 
curious ova attached to the conjunctiva of a turtle's eye; and 
when he afterwards submitted specimens to my scrutiny, I 
had no hesitation in pronouncing them to be referable to some 
particular species of ectozoon or entozoon belonging to one 
or other of the allied genera Polystoma , Tristoma , Octobothrium , 
and Bactylogyrus. Now, it is particularly noteworthy, that 
these several genera belong to the same great group of flukes 
or trematodes as the Bilharzia; but whereas, in the case of 
the African human blood-fluke, the ova display only a soli¬ 
tary and imperfectly developed holdfast, placed at one end of 
the shell, the singular eggs described by Mr. Canton are 
found to develope organs of anchorage at both extremities. I 
may remark, morever, as I also mentioned at the time, that 
parasitic ova exhibiting analogous processes, spines, and 
filamentary appendages at both poles, had already been 
observed by helminthologists in various species of parasite— 
as, for example, in Monostoma verrucosum infesting the fox, in 
