Roussel de Vauzème has mentioned another worm, a nematode, to which 45 he has given the name of _Odontobius_, and which lives on the palatal membranes (the whalebones) of the southern whale. It is evidently a messmate. It can get nothing from the whalebones, but it snaps up on their passage in the interstices of the baleen, small animals of all kinds which swarm in these waters. When we open the _Pylidium girans_, we often find in the interior of its digestive cavity a larva, which was once thought to be descended from it, but instead of being allied to the Pylidium, this larva comes from a nemertian known by the name of _Alardus caudatus_. The young nemertian never abandons his host until it approaches the period of puberty, and then all the individuals living under the same conditions emancipate themselves at once, to pass the rest of their days free and roving like their mother. Worms which have less freedom, like the Distomians, are sometimes both messmates and parasites. We find a remarkable example of this in the _Distomum ocreatum_ of the Baltic. According to the observations of Willemoes-Suhm, this trematode passes its cercarial life freely in the sea, and instead of encysting itself in the body of a neighbour, it attaches itself to a copepod crustacean, the whole of the inside of which it devours, in order to clothe itself afterwards with the carapace of its victim. It is under the cover of its prey that it passes into the herring, and completes its sexual evolution. Mons. Ulianin has recently found another Distome (_Distomum ventricosum_) which passes its cercarial life in freedom in the bay of Sebastopol, and completes its evolution in the fishes of the Black Sea. J. Müller has long since found Cercaria living freely in the Mediterranean. We ourselves, some years ago, while making some researches among the 46 Turbellaria, found among the eggs of some ordinary crabs of our coasts (_Carcinus mænas_), an interesting worm which we named _Polia involuta_, but which Prof. Kolliker appears to have known before, and designated by the name of _Nemertes carcinophilus_. It is not known whether it plays the same part as the Histriobdellæ and the Udonellæ. Delle Chiaie, as well as Prof. Frey and Prof. Leuckart, make mention of another nemertian which inhabits the _Ascidia mamillata_. Among the nemertians, we may allude to the _Anoplodium parasita_, which lives in the _Holothuria tubulosa_, and the _Anoplodium Schneiderii_, inhabiting the intestines of the _Stichopus variegatus_.