In the Bay of Massachusetts, on the coast of New England, another curious messmate lives at great depths; Dana has lately described it, under the name of _Epizoanthus Americanus_, V. It establishes itself in the _Eupagurus pubescens_. The _Sertularia parasitica_ of the gulf of Naples, from which I have formed the genus _Corydendrium_, is a messmate after the manner of an infinite number of other polyps. In closing this list, we shall mention a polyp, named _Halicondria suberea_, and the _Actinia carcinopodus_ of Otto, which inhabit an univalve mollusc; as also the _Heterosammiæ_ and the _Heterocyathi_ of the family of Turbinolidæ, which lodge in a trochoid shell. The sponges, placed by naturalists by turns among plants or on the confines of the animal kingdom, are now generally regarded as polyps; this is the opinion expressed by Haeckel, who wishes at the same time to replace the term Coelenterata by that of Zoophytes. The learned naturalist of Jena, when making this proposition, should have remembered that in 1859 we placed the sponges in the group of polyps, as the lowest in the scale; and that we proposed, from the time when the acalephæ were recognized to be adult polyps, to designate all these animals under the name of Polyps. Some time after, R. Leuckart proposed the appellation Coelenterate Polyps, which has been generally received. Professor Haeckel would have lost nothing by acknowledging that in 1873 he arrived at a result similar to that to which I had 64 come twenty years before, and that it is not a very happy innovation to change the term polyps for zoophytes. It is the more surprising that this naturalist has forgotten to quote my opinion, since at the congress of naturalists at Hanover in 1866, I had placed this question on the agenda for an ordinary meeting. I maintained, in opposition to the opinion of the naturalists whose authority had been especially recognized in the matter (Osc. Schmidt, who was present, among others), that sponges are lower polyps, whether they are regarded as to their development or their organization. This group, so remarkable in form, so varied in colour and appearance, very often affords examples of animals which live with them as true messmates; and we find the same relations established between them in both hemispheres. As we observe rhizophales on crabs and soldier-crabs, and pinnotheres on bivalve molluscs, so we find that the sponges of the Indian Seas or of Japan harbour the same messmates which we discover on them in the Northern Seas or the Atlantic.