Mons. Lacaze-Duthiers, who went to the coast of Africa to study corals, met with a young polyp which requires the assistance of another polyp in its early condition. This animal, to which he has given the name of _Gerardia Lamarckii_, lives on one of the Gorgoniæ, which it invades and stifles, as the lianas strangle the tree over which they spread themselves. But these same Gerardiæ can also develop themselves on the eggs of the _Plagiostoma_, and are then 50 capable of living separately. In the substance of this polyp lives a crustacean, the nature of which Mons. Lacaze-Duthiers has not yet made known. The superb sponge, _Euplectella aspergillum_, the elegant structure of which cannot be sufficiently admired, is, unlike the Alcyonium of the Dromia, rooted to the soil, but nevertheless gives shelter to three kinds of crustaceans: Pinnotheres, Palemonidæ, and Isopods. These supposed plants have been known for many years under the Spanish name of _Regadera_, or the English "Venus' Flower-basket;" they were first brought from Japan, and afterwards from the Moluccas, and more recently from the Philippine Islands. In almost all the individuals which Professor Semper was able to study in those parts, were found the same crustaceans. These _Euplectellæ_ have just been met with to the south-west of Cape St. Vincent, by Wyville Thomson, who has brought up some from a depth of 1090 fathoms, while on board the _Challenger_. This skilful professor has discovered another sponge to the north-west of Scotland, at a depth of 460 fathoms; it bears the name of _Holtenia Carpenteri_; and I have in my possession a fine specimen which I owe to his generosity, and keep as a _souvenir_ of the delightful hospitality which he extended to me at the Edinburgh meeting. There are also sponges which construct a dwelling in the abode of their neighbour. We find, among others, a small sponge known under the name of _Clione_, which establishes itself in the substance of the shell of oysters, and hollows out galleries as the teredo does in wood. Mr. Albany Hancock found twelve species of Clione on a single 51 Tridacna. They are evidently not parasites, and I am not sure if their place is properly among messmates. The oyster, and more especially the _Ostrea hippopus_, lodges three or four different sorts in its shell. These Cliones possess siliceous spicules, by means of which they hollow out galleries in the substance of shells. Mr. Hancock has published a monograph of this genus, in which he recognizes twenty-four species collected from different shells, and two other species, which he refers to the genus _Thoasa_.