Some fourteen years ago, Jacobson of Copenhagen wrote an interesting essay, to show that the young bivalves that are found in the branchiæ of anodonts at a certain period of the year are parasitical animals, for which he proposed a new name. But these supposed parasites are only young anodonts, which by the help of a very long cable, which proceeds from their foot like a byssus, attach themselves to their mother, or to a fish which will carry them to a distance. We see full-grown acephalous molluscs, as mussels and pinnæ, still keep these cables, under the name of byssus, during their whole life. There are also among distomians, worms which though they are hermaphrodite, couple two and two, and have this additional peculiarity, that while one increases rapidly the other becomes 84 atrophied. An Egyptian distome, which lives in man, gives an instance of this peculiarity, as well as the _D. filicolle_, which inhabits a fish (_Brama Raii_). The caligi which live on the skin of fishes are, when young, fastened by a cord which comes from the anterior edge of their carapace: while quite little, they put themselves under the protection of a kind neighbour, and allow themselves to be led by him. The new tubularia, which we have dedicated to our learned colleague Dumortier, often fixes itself on the carapace of ordinary crabs, and causes itself to be conveyed like the Echeneis; the tubulary observed by Gwyn Jeffreys, close by the eye of the _Rossia papillifera_, a cephalopod mollusc, perhaps belongs to the same species. Every colony of campanulariæ or sertulariæ lodges a crowd of messmates and mutualists; and there are a great number of crustaceans and polyps of all sizes which serve as an abode for infusoria of every kind. Some establish themselves on the carapace or on the swimming appendages, as in a carriage; others on one of the gills, which renders their mode of life more easy, and the danger less great. An amphipod very extensively spread over our sea-coasts, the _Gammarus marinus_, usually has its appendages covered with _Vayinicola crystallina_. CHAPTER V. 85 PARASITES. "En plongeant si bas dans la vie, je croyais y rencontrer les _fatalités physiques_, et j'y trouve la justice, l'immortalité, l'espérance."--MICHELET, _l'Insecte_.