Animal Parasites and Messmates

by P. J. van Beneden

The Picnogonons, the nature as well as the kind of life of which has been so long time problematical, deserve to be ranked among messmates, at least during their youth; in fact, after being hatched, they live on

the Corynes, the Hydractiniæ, and other polyps, while at a later period they frequent molluscs or higher classes; Allman mentions the case of a Phoxichilidium coccineum lodged in a Syncoryne.

There are, perhaps, many other crustaceans which, placed among messmates, like the Pandarus and others, would have a right to claim a further inquiry. It is a fact that they are never seen except on the skin of their host, where they are always visible, preserve their colours entire, and never change their costume for the undress of a parasite. The Pandari live especially on the Squalidæ. Some which are found in our seas are of rare elegance of form. We must, perhaps, place among messmates the crustacean which Siebold found in the Adriatic, at Pola, on the belly of the worm Sabella ventilabrum, and it is not impossible that the Staurosoma observed by Will on an actinia, should have its place here rather than among the parasites.

A Rotifer without vibratory ciliæ, the Balatro calvus of Claparède, lives as an epizoon on the same annelids which lodge the Albertia in their interior. The Darwinists, observes Claparède, will not fail to remark the presence of these Rotifers of the genus Albertia in the interior of the animal, and of the genus Balatro on the exterior. The parasite Balatro, like a shadow, never quits his Mecænas, says the learned naturalist of Geneva; who has observed it on the limicolous Oligochæts of the Seime, in the Canton of Geneva.

The Nebalia of Geoffroy is an interesting crustacean, abundant on the coast of Brittany. This charming animal gives lodging habitually to a messmate which Mons. Hesse considered as an animal allied to the

Histriobdellæ, but which is only an imperfectly described Rotator. We believe that it is the same animal to which Professor Grube has given the name of Seison nebalia. It appears to assume the aspect of the Histriobdellæ, and may perhaps be adduced as an example of mimicry.

The molluscs, whatever their name may imply, are those which show the most independence among all the inferior ranks of animals; not only are they contented with the slowness of their pace and the wretchedness of their food, but they only very rarely seek help from their neighbours. It is not, however, uncommon to find some living among corals, which have even been designated coralligenous molluscs. There exists a group of Gasteropods, the Eulimæ, which lodge in certain Echinoderms, and in every respect deserve to be classed among messmates; it was a long time before the relation which exists between them and the animals which shelter them had been thoroughly appreciated. Dr. Gräffe found one species, the Eulima brevicula, on the Archaster typicus of the Uvea Islands, in the Pacific Ocean. The molluscs, known by the name of Stylifer, have the same mode of life; they have been observed in the Asteriæ, the Ophiuræ, the Comatulæ, and even in the Holothuriæ; and as they inhabit the digestive cavity of these animals, it was believed that they frequented them as parasites. This was the opinion expressed first by d’Orbigny, and adopted by most naturalists. Professor Semper found some in the skin of a holothurian (Stichopus variegatus), which he considered incapable of nourishing themselves otherwise than at the expense of their host. However this may be, these molluscs,

ranged alternately among the Phasianellæ, the Turritellæ, the Cerithia, the Pyramidellæ, the Scalariæ, the Rissoairia, or in a distinct family, seem to belong rather to messmates than to parasites. We meet with Stylifers at the entrance of the mouth (Montacuta); more frequently they prefer, like the Fierasfers, to lodge themselves deeply in the digestive cavity in the midst of the débris of the prey. The Melania (M. Cambessedesii, Risso), which Delle Chiaie found in the Bay of Naples, on the foot of some comatulæ, belongs probably to this group of molluscs.

Among the gasteropod molluscs which are not able to maintain themselves, we may mention another, a curious parasite, which instals itself in one of the rays of a star-fish, and whose presence is revealed by a swelling which is not produced in the other rays. This mollusc has received the name of Stylina.

The molluscs which are the most remarkable from the point of view from which we are now considering them, are the Entoconchæ; they live in Enchinoderms, and it was thought for a while that we could see in them an example of the transformation of one class into another. Some years since J. Müller found in a Synapta from the Adriatic, tubes with male and female organs, without any other apparatus, and in these tubes appeared eggs, whence this great physiologist saw molluscs proceed, with a helicoid shell, similar to that of a small natica; he gave them the name of Entoconcha mirabilis. Professor Semper has since discovered another species of these, which he has dedicated to the illustrious physiologist of Berlin, and which he found attached to the cloacal sac of the Holothuria edulis.