Certain annelids off the coasts of Normandy and Bretagne are the abodes of a bryozoary known under the name of _Pedicellina_, or _Loxosoma_. This interesting animal, which my fellow-labourer, Mons. Hesse, took for a Trematode, and whose drawings had led me into error, lives like others at liberty while young, and soon fixes itself to a Clymenian, in order to pass as a messmate the later period of its life. We have called it _Cyclatella annelidicola_, because of 42 its residence in a Clymenian annelid. Claparède and Keferstein have observed a species, the _Loxosoma singulare_, on a capitellian annelid, of the genus _Notomastus_, at St. Vaast-la-Hogue, on the coast of Normandy. After this, Claparède found another species, the _Loxosoma Kefersteinii_, in the bay of Naples, on an _Acamarchis_, a bryozoarian mollusc. Mons. Kowalewsky has observed in the Bay of Naples the _Loxosoma Napolitanum_. We found some years ago the Pedicellinæ in so great abundance in the oyster beds of Ostend, that the baskets and other things floating on the water were literally covered with them. We have several times since endeavoured to procure them again, but it was in vain to search in the same places where they were formerly so abundant: we have not been able to discover a single one. The class of worms includes not only parasites, it contains also, as we shall see, true messmates; we find some on crustaceans, on molluscs, on animals of their own class, on Echinoderms, and on Polyps. One of the most curious of these worms is the _Myzostoma_, whose true nature has just been revealed by the excellent researches of Mons. Mecznikow. These myzostomes resemble trematode worms, but they have symmetrical appendages, and are covered with vibratory ciliæ. They live on the comatulæ, and run upon these echinoderms with remarkable rapidity. They have not hitherto been found elsewhere; they are evidently no more parasites than the last mentioned, and their place is among free messmates. Two great annelids are found, the one, the _Nereis bilineata_, by the side of Paguri in the same shell, the other, the _Nereis succinea_, according to Grube, in the tubes or galleries 43 of the Teredines. These dangerous acolytes introduce themselves furtively into the retreat of their host; and, always on the watch, they obtain at all times, and in every place, a certain prey, and a hiding-place from which they can take their share of their neighbour's goods. Another nereis, observed by Delle Chiaie, _Nereis tethycola_, lives in the cavities of a sponge, the _Tethya pyrifera_, which is visited by so many messmates and parasites, that it becomes a kind of hotel, where every one establishes himself at his ease. Risso also mentions a _Lysidice erythrocephala_ which lives in sponges.