Animal Parasites and Messmates

by P. J. van Beneden

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 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 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.

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 Chœtopterus 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 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.