TREMATODA 



41 



its apex always projects beyond the level of the curved end 

 of the pole. Now and then the spine is altogether absent (fig. 

 8 b) ; and when present it is, as already hinted, very 

 unequally developed. In size the spine ranges from a mere 

 point, having an extreme length of only soW', up to the 

 comparatively large magnitude of ^^' lengthways. 



According to the best evidence there is no good ground for 

 asserting the existence of any specific differentiation between 

 the parasites coming from the Cape and Egypt respectively. 



Taking a more extended view of the significance of these 

 singular chorional spines, I think we may here 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 deve- 

 lopment. To me it seems 

 that the little process in 

 question is a kind of rudi- 

 mentary holdfast ; and, as 

 such, it may be reckoned 

 as the homologue of a 

 variety of egg-appendages. 

 Eleven years ago Mr Ed- 

 win Canton discovered some curious ova attached to the 

 conjunctiva of a turtle's eye. I had no hesitation in pronounc- 

 ing them to be referable to some ectozoon or entozoon 

 belonging to one or other of the allied genera Polystoma, 

 Tristnma, Octobothrium, and Dactylogyrus. Now, whilst the 

 Bilharzia ova display only a solitary and imperfectly deve- 

 loped holdfast, placed at one end of the shell, the singular 

 eggs described by Mr Canton develop organs of anchorage at 

 both extremities. Parasitic ova exhibiting analogous pro- 

 cesses, spines, and filamentary appendages at both poles, 

 have been observed in various species of parasite as, for 

 example, in Monostoma verrucosum infesting the fox, in Ttenia 

 cyathiformis infesting the swallow, in Taenia variabilis of the 

 gambet, in Octobothrium, lanceolatum attached to the gills of 

 the common herring; and in Polystoma appendiculata, from 

 the branchiee of various marine fishes. Eggs of parasites which, 



Fro. 9. Two eggs of Bilharzia, with eccentrically placed 

 spines. That to the left shows mulberry cleavage of 

 the yolk ; the other having lost its embryonal contents 

 by rupture. Original. 



