The Adriatic also produces a species of the same genus (_Polythoa_) which inhabits, like that of the Chinese Sea, a sponge to which the name of _Axinella_ has been given. These Polythoæ are only found on the Axinellæ, says Osc. Schmidt, who has especially studied the sponges of this sea and of the Mediterranean. Professor Gill mentioned at the last meeting of the scientific congress at Portland (1873), a new Hyalonema found on the coast of North America by the fishery commission of the United States. A memoir on these sponges, interesting in a systematic point of view, is due to the pens of Herklots and of Marshall. [Illustration: Fig. 1.--Ophiodendrum abietinum on Sertularia abietina.] We think that we ought to place among fixed messmates a very problematical organism which lives on Sertulariæ, especially on the _Sertularia abietina_, and which Strethill Wright has designated by the name of _Corethria sertularia_. Claparède has given to this singular animal the more expressive name of _Ophiodendrum abietinum_. We have regularly found it on the _Sertularia abietina_ at Ostend, 67 every time that we have had an opportunity of observing these polyps immediately that they have been raised from the bottom of the sea. It is an organism whose affinities are not yet established. CHAPTER IV. 68 MUTUALISTS. In this chapter we bring together animals which live on each other, without being either parasites or messmates; many of them are towed along by others; some render each other mutual services, others again take advantage of some assistance which their companions can give them; some afford each other an asylum, and some are found which have sympathetic bonds which always draw them together. They are usually confounded with parasites or messmates. Many insects shelter themselves in the fur of the mammalia, or in the down of birds, and remove from the hair and the feathers the pellicle and epidermal _débris_ which encumber them. At the same time they minister to the outward appearance of their host, and are of great utility to him in a hygienic point of view. Those which live in the water have other guardians: instead of insects, we find a number of crustaceans which establish themselves on fishes, and if there are no scales of the epidermis which annoy them, there are mucosities which are incessantly renewed in order to protect the skin from the continual action of the water. We find many on the surface of the scales, and others which conceal 69 themselves at the bottom of mucous canals. We have brought together only a few examples, and there are certain others which are mentioned elsewhere, but which ought more properly to be placed here.