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.
Roussel de Vauzème has mentioned another worm, a
nematode, to which 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 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.
According to Mr. A. Agassiz, a species of Planarian (Planaria angulata, Mull.), lives as a free messmate on the lower surface of the Limulus, and prefers to establish itself near the base of the tail. Mons. Max Schultze recognized last year this same messmate on a limulus, which had died at Cologne in the large aquarium, and which had been sent to him for his anatomical studies. He showed at the congress of German naturalists at Wiesbaden, in 1873, the drawing which he had made of this animal, which he thought new to science. We may remark in passing, that he arrived, by means of his anatomical observations on Limuli, at the same result as did my son by his embryogenic observations, namely, that these supposed crustaceans are to be regarded as aquatic scorpions. Mr. Leidy also makes mention of Planarian parasites (Bdellura), with a sucker at the extremity of the body; and Mons. Giard noticed a blue one on the body of a Botryllus.
But of all the Turbellaria, the genus which appears to us the most interesting is the Temnophila, which Gay first observed on crabs at Chili, and which Professor Semper afterwards found on the crabs of the Philippine Islands. Gay and Phillipi found colonies of these animals on the body, the claws, and more especially the abdomen, of the Œglea. This messmate resembles a trematode by its form and by its posterior sucker, but by its entire character, and especially by its sexual organs, it belongs to the Turbellariæ. Mons. Blanchard calls it Temnophila Chilensis. Professor Semper saw at the Philippine Islands these Temnophilæ on river crabs, at five thousand feet above the level of the sea.
The Cydippe densa, a charming polyp of the Gulf of Naples, lodges in its gastro-vascular apparatus larvæ of annelids, which may as well be considered parasites as messmates. We owe to Panceri the first observations on these worms, of which two genera, Alciopina and Rhynconerulla, seem to live in the same manner in their youth. A naturalist, whose loss is profoundly deplored by the scientific world, Claparède, occupied himself with observations on these annelids during the last years of his life. It appears that these worms are so common in these polyps, that four have been found at once in the same animal.
The Spoon-worm, named by Œrsted, Sipunculus concharum, ought doubtless to find its place here. An oligochete worm, Hemidasys agaso, from the Gulf of Naples, lives on the Nereilepas caudata, and Claparède did not think it unworthy of his attention. The surest means of finding it, says this philosopher, is to look for it on this annelid; and our much regretted fellow-labourer