I close this series of isolated examples with an observation which indeed only half belongs to the province of the Crustacea to which these pages ought to be confined, and which also has no further connexion with the preceding circumstances than that of being an “intelligible and intelligence-bringing fact” only from the point of view of Darwin’s theory. To-day as I was opening a specimen of _Lepas anatifera_ in order to compare the animal with the description in Darwin’s ‘Monograph on the Subclass Cirripedia,’ I found in the shell of this Cirripede, a blood-red Annelide, with a short, flat body, about half an inch long and two lines in breadth, with twenty-five body-segments, and without projecting setigerous tubercles or jointed cirri. The small cephalic lobe bore four eyes and five tentacles; each body-segment had on each side at the margin a tuft of simple setæ directed obliquely upwards, and at some distance from this, upon the ventral surface, a group of thicker setæ with a strongly uncinate bidentate apex. There was above _each_ of the lateral tufts of bristles a branchia, simple on a few of the foremost segments, and then strongly arborescent to the end of the body. The animal, a female filled with ova, evidently, from these characters, belongs to the family of the Amphinomidæ; the only family the members of which, being excellent swimmers, live in the open sea. That this animal had not strayed accidentally into the _Lepas,_ but appertained to it as a regular and permanent guest, is evidenced by its considerable size in proportion to the narrow entrance of the test of the _Lepas,_ by the complete absence of the iridescence which usually distinguishes the skin of free Annelides and especially of the Amphinomidæ, by the formation and position of the inferior setæ, etc. But that a worm belonging to this particular family Amphinomidæ living in the high sea, occurs as a guest in the _Lepas,_ which also floats in the sea attached to wood, etc., is at once intelligible from the stand-point of the Darwinian theory, whilst the relationship of this parasite to the free-living worms of the open sea remains perfectly unintelligible under the supposition that it was independently created for dwelling in the _Lepas._ But however favourable the examples hitherto referred to may be for Darwin, the objection may be raised against them, and that with perfect justice, that they are only isolated facts, which, when the considerations founded upon them are carried far beyond what is immediately given, may only too easily lead us from the right path, with the deceptive glimmer of an _ignis fatuus._ The higher the structure to be raised, the wider must be the assuring base of well-sifted facts.