The _Philomedusa Vogtii_ of Fr. Müller, which lives on the _Halcampa Fultoni_, undoubtedly deserves to be mentioned here as a fixed messmate. Many bryozoa spread themselves over marine animals, and often engage in a deadly struggle with their patron. But among all these bryozoa we must mention an animal very common on the sea-shore at Ostend, and which one would take for a dried leaf, the _Flustra membranacea_. On the surface of these imitative leaves are found little bouquets of other bryozoa, which are either _Crisiæ_ or _Scrupocellariæ_. Another kind, which has also passed for a gelatinous plant, bears the name of _Halodactylus_. Without any microscopic study, one can obtain an idea of these colonies. One of these 62 Halodactyles spreads itself upon the stalk of a Sertularia, all the inhabitants of which it stifles, so that it is the victim himself who serves as a guardian to the invader. These Halodactyli are very widely spread over the Northern Seas, and often establish themselves on the large horse-hoof oyster. Michelin has noticed under the name of parasite a fossil _cellepore_ from the saltpits of Touraine and Anjou, which entirely surrounds the shell of a gasteropod; in order to prevent its patron from dying of hunger, the bryozoon develops itself around the mouth like a gallery, and prolongs its last spiral. This _Cellepora parasitica_ has evidently a place here. Many of these messmate bryozoa are found in a fossil state in the crag of the Antwerp basin. We have still to mention among fixed messmates many polyps, some of which are very remarkable. Thus, many naturalists speak of vast colonies of polyps in which lodge various animals which shelter themselves there like paguri in deserted shells. Among these are the colonies of which Forster speaks, which are not less than three feet in diameter, and fifteen feet in height, with a crown of eighteen feet in diameter. Dana also makes mention of an _Astræa_ of twelve feet in height, and of _Porites_ twenty feet high, which contain more than five millions of individuals, among which a number of animals come to take refuge. The Museum of Natural History at Paris is in possession of a superb specimen of _Porites conglomerata_: in the middle of the colony lodges a Tridacna (_Trid. corallicola_, Val.) like a pagurus under a forest of hydractiniæ. This remarkable polyp was brought from the Seychelles Islands by Mons. L. Rousseau. It is not impossible that pinnotheres 63 live in this same tridacna, and that we have there a fresh example of messmate within messmate.