The shell is transparent, of a brown colour, and free from any markings, lines, or sculpturing. One pole of the shell is invariably narrower than the other, and usually presents a more or less pointed extremity (fig. 8). This narrow end commonly displays a sharp, projecting, beak-like spine, which, at its base, constantly rests upon the centre of the pole of the shell, but occasionally it is eccentrically placed (fig. 8_a_). In some few examples the spine is removed to a little distance from the actual extremity of the shell; but even in these instances, so far as my observations go, 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 1/8000″, up to the comparatively large magnitude of 1/2500″ 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. [Illustration: FIG. 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.] 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 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. I had no hesitation in pronouncing them to be referable to some ectozoon or entozoon belonging to one or other of the allied genera _Polystoma_, _Tristoma_, _Octobothrium_, and _Dactylogyrus_. Now, whilst the Bilharzia ova display only a solitary and imperfectly developed 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 processes, 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 _Tænia cyathiformis_ infesting the swallow, in _Tænia variabilis_ of the gambet, in _Octobothrium lanceolatum_ attached to the gills of the common herring; and in _Polystoma appendiculata_, from the branchiæ of various marine fishes. Eggs of parasites which, like _Bilharzia_, are furnished with a single appendage, may likewise be seen in the ova of different species of _Dactylogyrus_ infesting the gills of the pike. In the more strongly