Cases of commensalism among the Echinodermata are still more rare. These animals are sufficiently provided with organs, both with respect to their food and their skin, not to require the assistance of their neighbours. We cannot rank as a phenomenon of commensalism, the conduct of the young Comatulæ, which fasten themselves, as Mr. A. Agassiz informs me, to the basal cirrhi of the adult echinoderms, and there form a little colony of young Pentacrinites. We only know one Ophiurus (_Ophiocnemis obscura_), which lives as a messmate on a comatula, and consequently seeks assistance from an animal of its own rank. Another kind of Ophiuride (_Asteromorpha lævis_, Lym.) fixes itself on a _Gorgonella Guadelupensis_ of Barbadoes. Everything induces us to suppose that we shall find more than one species of echinoderm, which will take its place among these when their mode of life has been studied with greater care. Professor Lütken has just proved this by quite recently making known another _Ophiothela_, which lives in the straits of Formosa, and seems to be the messmate of an Isidian polyp, known under the name of _Parisis loxa_. Another species (_Oph. mirabilis_) from Panama, infests certain Gorgoniæ and sponges; a third is found in the Fiji Islands on the _Melitodes virgata_; a fourth at the Isle of France on Gorgoniæ; and a fifth at Japan on the _Mopsella Japonica_. There is also another in the Pacific Ocean, but its companion is not known. Professor Mobius, as well as Dr. F. Martens, has noticed a _Hemicuryale 49 pustulata_ on a polyp of Jamaica, known under the name of _Verrucella Guadelupensis_. This is a curious instance of mimicry. The class of polyps includes several species which seek for assistance from others, and are classed among messmates. One of the most remarkable is the Gigantic Medusa, which can extend its arms downwards to a hundred and twenty feet, and bears the name of _Cyanea arctica_; the disc is seven feet and a half in diameter, and when the animal is on the surface of the water, the fringes, which surround the cavity at its mouth, occasionally afford lodging in the midst of them to a species of actinia, which lives there as messmate. Sometimes three, and even four or five, are found on a single Cyanæa. This also is an observation due to Mr. A. Agassiz, which he has published in his interesting work, "Sea-side Studies." Prof. Haeckel supposed that the _Geryoniæ_ produce _OEginidæ_ by means of buds; but it appears that the learned professor was mistaken as to the nature of these buds; that instead of being produced one from the other, they have, according to Steenstrup, a completely different genealogy, being only united by conditions of good-fellowship. They may be truly called messmates.