There are cirrhipedes on the gasteropod molluscs. The _Concholepas Peruviana_, that beautiful shell which has long been considered a rarity in our collections, is frequented by the _Cryptophiolus minutus_, only a sixth of an inch in length. The _Scalpella_ often inhabit the Sertulariæ and other polyps; _Oxynasps_, _Creusiæ_, _Pyrgomæ_, and _Lithotryæ_ inhabit corals. Certain kinds of sponges are regularly invaded by the _Acastæ_ of Leach, eight species of which are mentioned by Darwin. As we find elsewhere parasites on parasites, here also we find messmates on messmates; on the common anatifa we perceive other genera, and on the _Diadema_ of the North Pacific, we almost always see _Otions_ and _Cineras_. The _Protolepas bivincta_ also, a fifth of an inch in length, lives as a messmate in the mouth of the _Alepas cornuta_; and the _Elminius_ of Leach also inhabits other cirrhipedes. The _Hemioniscus balani_, which Goodsir had taken some years ago for the male of the Balanus, is a messmate on these cirrhipedes. Parasites also are found in messmates; the soldier-crab gives lodging to the sexual _Eustoma truncata_ in its interior. 61 A macrourous crustacean which we ought to mention here, the _Galathea spinirostris_, Dana, frequents a comatula, the colour of which it assumes; it is the same without doubt with the _Pisa Styx_, which lives on a polyp known by the name of _Melitoea ochracea_. If we pass from the crustaceans to the molluscs, we have to notice in the first place an elegant gasteropod, the _Phyllirhoa bucephala_, which carries on its head a singular appendage, the nature of which has only lately been known; J. Müller took it at first for a medusa, then he abandoned this opinion, when at length Mons. Krohn referred it definitively to the lower polyps; it differs from its congeners only by its form, its tentacular cirrhi, and its mode of life: it is the _Mnestra parasites_. There are a great number of acephalous molluscs, which we might mention as messmates, but we will only refer to the _Crenellæ_ which are regularly found in the substance of sponges.