Among the molluscs, the Tunicates give lodging to the greater number of lerneans; in the cavity which is before the mouth, and by which the food passes, some are found which can scarcely be recognized, and which remain there to smell out a feast. The _Aplidium_ of the coasts of Belgium gives lodging to some which are very curious, and which we have named _Enterocola fulgens_, on account of their colours. The _Notopterophorus_ establishes itself on the body of the _Phallusia mamillaris_, and a certain number of these parasites are found on the annelids. Professor Sars of Christiania, and Claparède have carefully 152 described them; and the latter saw on the _Spirographis Spallanzani_ of the bay of Naples, a female which he called _Sabelliphilus Sarsii_. The genera _Selius_, _Silenium_, _Terebellicola_, _Chonephilus_, _Sabellacheres_, _Nereicola_, &c. infest all the annelids; the _Eurysilenium truncatum_ lives on the _Polinoë impar_, the _Melinnacheres ergasiloïdes_ on the _Melinna cristata_. The echinodermata and the polyps are not free from lerneans; thus the _Asterochoeres Lilljeborgii_ fixes itself on the _Echinaster sanguinolentus_, and we have found a very beautiful species in Brittany on an Ophiurus; the _Loemippa rubra_, allied to the _Chondracanthi_, lives upon the _Pennatula rubra_, the _Laura Girardiæ_, according to Mons. Lacaze Duthiers, feeds on an Antipathes. A Loemippus (_Proteus_) lodges in the cavity of the body of the _Lobularia digitata_ of Delle Chiaie; and lastly, the _Enalcyonium rubicundum_ is sheltered by the _Alcyonium digitatum_. There are certain worms which are free when young, and only become parasites at a later period of their evolution. We will give a few examples.