I cursorily examined the cementing apparatus in _Balanus galeatus_, _improvisus_ and _crenatus_, which have all calcareous bases, and belong to different sections of the genus; and the structure seemed to be essentially the same. In _Bal. galeatus_, I found the cement-ducts varying in diameter from 1/4000th to 1/10,000th of an inch in diameter. In _B. improvisus_, the cement-glands do not differ much from those of _B. tintinnabulum_; but the cement-ducts bifurcate often before entering the circumferential duct; and the little branches, which proceed from the latter, are very short, and almost immediately, owing to the thinness of the basis, blend into a slip of cement. * * * * * I hope to be excused for describing at such length, the apparatus by which sessile cirripedes are permanently attached to a supporting surface; for this is the great leading character of the sub-class, not hitherto observed in any other Crustacean.[73] It is not easy to overstate the singularity and complexity of the appearance of the basal membrane of a Balanus or Coronula: and when we consider the homological nature of the apparatus, the subject becomes still more curious: I feel an entire conviction, from what I have repeatedly seen in several genera of the Lepadidæ, both in their mature and pupal condition, and from what I have seen in Proteolepas, that the cement-glands and ducts are continuous with and actually a part of an ovarian tube, in a modified condition; and that the cellular matter which, in one part, goes to the formation of ova or new beings, in the other and modified part, goes to the formation of the cementing tissue. To conclude with an hypothesis,--those naturalists who believe that all gaps in the chain of nature would be filled up, if the structure of every extinct and existing creature were known, will readily admit, that Cirripedes were once separated by scarcely sensible intervals from some other, now unknown, Crustaceans. Should these intervening forms ever be discovered, I imagine they would prove to be Crustaceans, of not very low rank, with their oviducts opening at or near their second pair of antennæ, and that their ova escaped, at a period of exuviation, invested with an adhesive substance or tissue, which served to cement them, together, probably, with the exuviæ of the parent, to a supporting surface. In Cirripedes, we may suppose the cementing apparatus to have been retained; the parent herself, instead of the exuviæ, being cemented down, whereas the ova have come to escape by a new and anomalous course.