We have found both at Marseilles and at Trieste, says J. Müller, a new cercaria with a pinnate tail, and two black ocular points; its body is from one-tenth to one-sixth of a line in length, not including the tail, which is twice or two-and-a-half times as long. There is a protuberance just in front of the middle of the body. At each side of the tail there are from twelve to twenty pencils of soft bristles placed on little prominences in a transverse series of six tufts, not regularly opposed to each other. In one specimen, the tail, from its point of insertion to the posterior quarter, is provided with these bundles of bristles; and in another they are wanting entirely in the anterior half, but exist, on the contrary, on the hinder half. In a third, the bristles have partially disappeared, and are reduced to six bundles at the extremity of the tail. This tail presents traces, more or less distinct, of transverse rings. J. Müller has often seen that the distome, which proceeds from this cercaria, swims freely in the sea, and after having got rid of its tail, could be easily recognized by the two black marks which were then more diffused. This cercaria described by J. Müller recalls to us that which was noticed by Nitzsch on fresh-water shells (_Cercaria major_) with an annulate and pinnated tail. Claparède also took at Saint-Vaast, cercariæ the host of which he did not know. This naturalist supposed that this worm could migrate at will. He found there the same cercaria (_C. Haimeana_) on Sarsiæ and Oceaniæ, but always sexless. The _Cercaria setifera_ of J. Müller has been found free and attached 194 to the lower surface of some medusæ. It exists occasionally in considerable numbers on the internal surface of some Acalephæ of the ocean and of the Mediterranean. Claparède has also observed another free cercaria which bears the name of _Pachycerca_. Some of the cercariæ are very tenacious of life; we have kept some alive in fresh water during a whole week in the month of November, and on the last day they were still active (_Cercaria armata_). We sometimes find the cercarian age passed over, and the young distomes appear abundantly without tails in the sporocyst. We have seen an example of this in the _Buccinum undatum_ of our coasts. This latter generation assumes in every case a very different form from that which preceded it.