_Fig._ 19.a. This Sea-Fan, with the Barnacles inclosed in it, was brought from Gibraltar. I have called it the Slipper Barnacle, from its shape. _See Fig._ 19. These shell-fish adhere, while they are young, to the slender branches, which are produced by the animals that compose this species of Sea-fan; and as the next succession of young animals of this sea-fan creep up its sides, to increase the bulk and extension of these first-formed ramifications, they inclose the shells all round, leaving only their mouths or apertures open, for the Barnacles to procure their food. But it frequently happens, that the animals of the Sea-fans destroy these Barnacles, by overrunning and involving them in the very center of their stems. These small Barnacles, interspersed here and there on the branches, have been taken for fruit or berries by some gentlemen, who look upon the internal or horny part of the Sea-fans to be vegetables. _Fig._ 20. is a very curious Barnacle, taken from an elegant specimen in the British Museum; which, from its figure, I have called the Persian Crown. I shall now add some further observations on the nature of these animals. Upon opening the shells of many of the common English Barnacles (_Fig._ 1.) while they were alive, I found the lower part of the shell, which contained a cavity equal to two thirds of the whole, full of spawn; so that the Barnacles, which adhere by the base of their shells, as well as those that are supported by fleshy tubes, are propagated by eggs, which they send forth in inconceivable numbers; as appears by the clusters of young shells, which we find adhering not only to the parent animals, but to all hard substances near them. The bottom shell of these animals, as well as their upper shells, vary in form according to their situation, which occasions some difficulty in determining their several species with exactness. The form of the base shell of our common English Barnacle, is the flat radiated figure represented adhering to a scallop shell in the front of a group of them at _Fig._ 17. The Barnacles at _Fig._ 8. 9. 14. 15. and 20. have the same kind of base. I have very lately observed a singular kind of flat Balanus, on a white Mandrepora coral from the coast of Italy, in the possession of Mr. Mendez D’Acosta, F.R.S. whose base appears sunk into the coral, and of the form of an inverted cone, bending a little to one side. The inward surface of this conical base shell appears curiously striated with tubular radii, which terminate on the surface of the coral, to receive the extremities of the six valves, that compose the upper shell. This peculiar form of the base seems owing to the animals of the coral and of the Barnacle growing up together, the latter keeping possession of its proper space, while the former grew close about it.