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. 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 Melitœa 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.
The Philomedusa Vogtii of Fr. Müller, which lives on the Halcampa Fultoni, undoubtedly deserves to be mentioned here as a fixed messmate. Many bryozoa spread themselves over marine animals, and often engage in a deadly struggle with their patron. But among all these bryozoa we must mention an animal very common on the sea-shore at Ostend, and which one would take for a dried leaf, the Flustra membranacea. On the surface of these imitative leaves are found little bouquets of other bryozoa, which are either Crisiæ or Scrupocellariæ. Another kind, which has also passed for a gelatinous plant, bears the name of Halodactylus. Without any microscopic study, one can obtain an idea of these colonies.
One of these Halodactyles spreads itself upon the stalk of a Sertularia, all the inhabitants of which it stifles, so that it is the victim himself who serves as a guardian to the invader.
These Halodactyli are very widely spread over the Northern Seas, and often establish themselves on the large horse-hoof oyster. Michelin has noticed under the name of parasite a fossil cellepore from the saltpits of Touraine and Anjou, which entirely surrounds the shell of a gasteropod; in order to prevent its patron from dying of hunger, the bryozoon develops itself around the mouth like a gallery, and prolongs its last spiral. This Cellepora parasitica has evidently a place here.
Many of these messmate bryozoa are found in a fossil state in the crag of the Antwerp basin.
We have still to mention among fixed messmates many polyps, some of which are very remarkable. Thus, many naturalists speak of vast colonies of polyps in which lodge various animals which shelter themselves there like paguri in deserted shells.
Among these are the colonies of which Forster speaks, which are not less than three feet in diameter, and fifteen feet in height, with a crown of eighteen feet in diameter. Dana also makes mention of an Astræa of twelve feet in height, and of Porites twenty feet high, which contain more than five millions of individuals, among which a number of animals come to take refuge.
The Museum of Natural History at Paris is in possession of a superb specimen of Porites conglomerata: in the middle of the colony lodges a Tridacna (Trid. corallicola, Val.) like a pagurus under a forest of hydractiniæ. This remarkable polyp was brought from