In the same class is found an Amphinoma, a beautiful red-blooded worm, which proudly wears a plume of red branchiæ on its head, and which Fritz Müller observed on the coast of Brazil, begging assistance from a poor _Lepas anatifera_. Many Polynoës live upon other annelids; the _Harmothoë Malmgreni_ on the sheath of the _Choetopterus insignis_, the _Antinoe nobilis_ on the case of the _Terebella nebulosa_. Prof. Ray Lankester has lately communicated some observations on this subject to the Linnæan Society of London, and Dr. M'Intosh mentions some new species leading the same kind of life on the coast of Scotland. Grube found at Trieste, in a star-fish (_Astropecten aurantiacus_), between its rows of suckers, a _Polynoë malleata_, with its stomach attached to the animal; and Delle Chiaie has lately observed on an asteria, a _Nereis squamosa_ by the side of a _Nereis flexuosa_. Mons. Grube thinks that the nereis of Delle Chiaie is no other than the _Polynoë malleata_. Lobsters are often covered with very small tubicular worms, which invade the whole carapace, and which, as true 44 messmates, give themselves up to the caprices of their host. These are a kind of _Spirorbis_, which, under the form of small spiral tubes, instal themselves, by preference, on the limbs, the antennæ, or the claws. Mr. A. Agassiz has seen on the coast of the United States, a Beroë (_Mnemiopsis Leidyi_) which gives lodging in its interior to worms which somewhat resemble the Hirudinidæ, and which doubtless live there as messmates. Mr. A. Agassiz has remarked to me another example of commensalism. On the coast of the territory of Washington, as far as California, is found a worm of the genus _Lepidonotus_, which always lives near the mouth of a star-fish, the _Asteracanthion ochraceus_ of Brandt; sometimes as many as five are found together on a single individual, and are placed on different parts of the ambulacral rays. Mr. Pourtalis and Mr. Verril have observed annelids lodged in the polypidoms of the Stylaster. There are few fish on which are not found _Caligi_, charming crustaceans which please the eye by their attenuated shape and their graceful movements. On these Caligi, which sometimes literally cover the skin of cod-fish coming from the north, we often find a curious trematode, the _Udonella_, which resembles one of the small hirudinidæ. Should this worm be placed among messmates? What is the part which it plays? We are persuaded that it is the same as that of the histriobdellæ under the tail of lobsters, that is to say, that it clears off the eggs of caligi which do not arrive at perfection, but perish in the course of their evolution.