If the forms are not so varied among the isopods as elsewhere, many among them present nevertheless the most extraordinary appearance, the most unexpected contour. Most of the parasitic isopods instal themselves in the thoracic cavity under the carapace of a neighbour, and make themselves contented in the small space which remains to them. After having disposed of their luggage, they arrange themselves scrupulously according to the extent of the lodging which they occupy, and, rather than interfere with the branchiæ, they raise up the walls of the cephalothorax, thus forming a sort of tumour which betrays the presence of the intruder. Others are found which are not contented with a natural cavity; they raise the scale of the skin of a fish, perforate or hollow out the true skin, or even pierce through the walls of the abdomen, in order to establish themselves in the 144 intestines, still keeping up a communication with the exterior. A very common species of this class is called _Bopyrus_. We often see beautiful prawns, which are usually remarkable for their fine rose colour, exposed for sale in shop windows. If we examine them at certain seasons, especially in France, we perceive that the carapace at the side is raised; and if we take it off with some precaution, we discover underneath an irregular flattened body, which fishermen take for a young sole on account of its shape. This is the female bopyrus. The many appendages of the thorax, the division into rings, the symmetry of the body, all have disappeared, and the claws, the traces of which are scarcely seen, are no longer similar on the right and left sides. The male remains small and independent, and preserves the livery of the order to which he belongs. On the coast of Labrador, a bopyrus behaves in the same manner towards a Mysis. We have found under the carapace of a pagurus a female bopyrus full of eggs, so much flattened that it might have been taken for a leaf accidentally introduced into this cavity. Fritz Müller has divided the Bopyridæ in the following manner:-- 1. Those which fix themselves on the appendages or in the branchial cavity of decapods; these are the Bopyri, Iones, Phryxi, Gyges, Athelgi, &c. 2. Those which live in the thoracic cavity of some Brachyuri, as the _Entoniscus_. 3. Those which live in the cirrhipeds, like the _Cryptoniscus_, as well as the _Liriopes_. 4. Those which live on copepods as true parasites, as the _Microniscus_ (_M. Fuscus_). The _Iones thoracicus_, the _Cepes distortus_, the _Gyges 145 branchialis_, and so many others live, like the Bopyri, in the thoracic cavity of different decapod crustaceans, and the females throw off at the same time their organs of sense and all their fishing and travelling apparatus.