An interesting division of amphipods have received the name of _Hyperinæ_. These crustaceans generally swim with facility, but walk with difficulty. They therefore usually have recourse to fishes, or even to medusæ, in order to gain support. We find on our own coasts the _Hyperina Latreillii_, lodged in the superb _Rhizostoma_, which 33 regularly appears in the later season of the year on the coast of Ostend; and a long time since, in 1776, O. F. Müller gave to a species of this genus the name of _Hyperina medusarum_. Mr. Alexander Agassiz once found a _Hyperina_ on the disc of an _Aurelia_. The medusa, when extended, forms for them a balloon with its parachute, which supports and conveys them with greater or less rapidity. Professor Möbius has but lately remarked the presence of _Hyperina galba_, Mont., in the _Stomobrachium octocostatum_, Sars, a small species of medusa which appears in the Bay of Kiel in October and November. This naturalist supposes that these messmates at first inhabited the _Medusa aurita_, and then migrated into this species. Besides these, there are _Gammari_, which, according to Semper, live in the _Avicula meleagrina_ (pearl mussel), and are perhaps the principal manufacturers of fine pearls. The immense buccal cavity of the fishing-frog (_Lophius piscatorius_) is the abode in the Mediterranean of an _Apterychta_, and in the Northern Ocean of a curious amphipod of the ordinary size of the _Gammarus_, which takes a voyage without expense, and with no fear of wanting provisions. My son discovered it at Ostend, and proposes the name of _Lophiocola_ to distinguish it. The Gammari give lodging themselves to a great quantity of parasites, which they must introduce into the bodies of those to whom they serve as food. It has been long known that whales have lice, to which naturalists have given the name of Cyami. They are found on the whales of both hemispheres, and on some other cetaceans. It is very remarkable that they are seen on the true whales of 34 the north and of the temperate regions, on the _Megaptera_, and on several _Catodonta_, and that none are found in the _Balenoptera_. Mr. Dall has just noticed some on the singular _Grey Whale_ of California. In general, we may say that each cetacean which harbours them, has its own species. Are they parasites or messmates? If we are to believe Roussel de Vauzème, they feed on the skin itself of the whale, the remains of which, it is said, are found in their stomach. According to this naturalist, the parts of the mouth are not adapted for suction, and the stomach contains ruminating apparatus. We think that a fresh examination is necessary before this question can be determined. The _Cyami_ seem to us to live on the whale, as the _Arguli_ and the _Caligi_ do on fish; and if these living creatures derive their nourishment only from the mucous products secreted by the skin, we may ask whether they ought not to be classed in a separate category, for