Rathke, a learned professor of Königsberg, was the first to notice an isopod, known under the name of _Phryxus paguri_, which lives on the stomach of a pagurus, attached to it by its back, so that the stomach of the parasite is turned, like that of the pagurus, towards the partitions of the shell. The tail with the branchial appendages is always directed towards the orifice of the shell. The male is very small and never leaves the female. The _Athelca cladophora_ is another bopyrian living on the abdominal region of a pagurus, which always chooses shells infested by Alcyonia. Another bopyrian, the _Prosthetes cannelatus_, lives on the abdomen of an ordinary pagurus. [Illustration: Fig. 29.--Phryxus Rathkei. A figure of the natural size is given at the side.] Mons. Bucholz has recently described a new kind of isopod, allied to the lyriopes, which lives on the _Hemioniscus_. This isopod fixes itself to a Balanus (_B. ovularis_), and the female preserves only four of her segments with their appendages: she had fifteen, when young. Thus she throws off nearly all her appendages which have become useless. The male of this isopod, which inhabits the bay of Christiansand, is not yet known. Another parasite of this group has been observed by Fr. Müller at Desterro, on the coast of Brazil. It bears the name of _Entoniscus porcellanæ_. The parasite which he 146 he discovered by the side of it on the same animal, and to which he has given the name of _Lerneoniscus_, had perhaps introduced it. We have seen examples of this kind among insects. Among the rich materials which Professor Semper brought back from his voyage, there was a Porcellana, which harbours on its exterior surface a very remarkable isopod, whose recurrent development is no less decided than that of the peltogasters. Dr. Kausmann has lately described these curious organisms, to which he has given the name of _Zeuxo_. Another isopod, with a no less decided recurrent development, has received from the same naturalist the name of _Cahira Lerneodiscoïdes_.