I must preface this with a few words upon the homology of the roots of the Rhizocephala, _i.e._ the tubules which penetrate from its point of adhesion into the body of the host, ramify amongst the viscera of the latter, and terminate in cæcal branchlets. In the pupæ of the Rhizocephala (Fig. 58) the foremost limbs (“prehensile antennæ”) bear, on each of the two terminal joints, a tongue-like, thin-skinned appendage, in which we may generally observe a few small strongly refractive granules, like those seen in the roots of the adult animal. I have therefore supposed these appendages to be the rudiments of the future roots. A perfectly similar appendage, “a most delicate tube or ribbon,” was found by Darwin in free-swimming pupæ of _Lepas australis_ on the last joints of the “prehensile antennæ.” From the perfect accordance in their entire structure shown by the pupæ of the Rhizocephala and Cirripedia, there can be no doubt that the appendages of _Sacculina_ and _Lepas,_ which are so like each other and spring from the same spot, are homologous structures. Now in three species of _Lepas,_ in _Dichelaspis Warwickii_ and in _Scalpellum Peronii,_ Darwin saw, on tearing recently-affixed animals from their point or support, that a long narrow band issued from the same point of the antennæ; its end was torn away, and in _Dichelaspis,_ judging from its ragged appearance, it had attached itself firmly to the support. From this it follows that this appendage in _Lepas australis_ can hardly be anything but a young cement-duct. If, therefore, the supposition that the appendages on the antennæ of the pupæ of Rhizocephala are young roots be correct, the roots of the Rhizocephala are homologous with the cement-ducts of the Cirripedia. And this, strange as it may appear at the first glance, seems to me scarcely doubtful. It is true that the act of adhesion of the Rhizocephala has never yet been observed, but it is more than probable that they attach themselves, just like the Cirripedia, by means of the antennæ, and that therefore the points of attachment in the two groups indicate homologous parts of the body. From the point of attachment in the Rhizocephala the roots penetrate into the body of the host, whilst in the Cirripedia, the cement-ducts issue from the same point. The roots are blind tubes, ramified in different ways in different species. The cement-ducts in the basis of the Balanidæ likewise constitute a generally remarkably complicated system of ramified tubes, with regard to the mode of termination of which nothing certain has yet been made out. Individual cæcal branches are not unfrequently seen even in the vicinity of the carina; and, at least in some species, in which the cement-ducts divide into extremely numerous and fine branchlets, forming a network which gradually becomes denser towards the circumference of the basis, these seem nowhere to possess an orifice.