Professor Mobius, as well as Dr. F. Martens, has noticed a Hemieuryale pustulata on a polyp of Jamaica, known under the name of Verrucella Guadelupensis. This is a curious instance of mimicry.

The class of polyps includes several species which seek for assistance from others, and are classed among messmates. One of the most remarkable is the Gigantic Medusa, which can extend its arms downwards to a hundred and twenty feet, and bears the name of Cyanea arctica; the disc is seven feet and a half in diameter, and when the animal is on the surface of the water, the fringes, which surround the cavity at its mouth, occasionally afford lodging in the midst of them to a species of actinia, which lives there as messmate. Sometimes three, and even four or five, are found on a single Cyanæa. This also is an observation due to Mr. A. Agassiz, which he has published in his interesting work, "Sea-side Studies." Prof. Haeckel supposed that the Geryonia produce Eginidæ by means of buds; but it appears that the learned professor was mistaken as to the nature of these buds; that instead of being produced one from the other, they have, according to Steenstrup, a completely different genealogy, being only united by conditions of good-fellowship. They may be truly called messmates.

Mons. Lacaze-Duthiers, who went to the coast of Africa to study corals, met with a young polyp which requires the assistance of another polyp in its early condition. This animal, to which he has given the name of Gerardia Lamarckii, lives on one of the Gorgoniæ, which it invades and stifles, as the lianas strangle the tree over which they spread themselves. But these same Gerardiæ can

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also develop themselves on the eggs of the Plagiostoma, and are then capable of living separately. In the substance of this polyp lives a crustacean, the nature of which Mons. Lacaze-Duthiers has not yet made known.

The superb sponge, Euplectella aspergillum, the elegant structure of which cannot be sufficiently admired, is, unlike the Alcyonium of the Dromia, rooted to the soil, but nevertheless gives shelter to three kinds of crustaceans: Pinnotheres, Palemonidæ, and Isopods. These supposed plants have been known for many years under the Spanish name of Regadera, or the English "Venus' Flower-basket; " they were first brought from Japan, and afterwards from the Moluccas, and more recently from the Philippine Islands. In almost all the individuals which Professor Semper was able to study in those parts, were found the same crustaceans. These Euplectellæ have just been met with to the south-west of Cape St. Vincent, by Wyville Thomson, who has brought up some from a depth of 1090 fathoms, while on board the Challenger. This skilful professor has discovered another sponge to the north-west of Scotland, at a depth of 460 fathoms; it bears the name of Holtenia Carpenteri; and I have in my possession a fine specimen which I owe to his generosity, and keep as a souvenir of the delightful hospitality which he extended to me at the Edinburgh meeting.

There are also sponges which construct a dwelling in the abode of their neighbour. We find, among others, a small sponge known under the name of Clione, which establishes itself in the substance of the shell of oysters, and hollows out galleries as the teredo does in wood.

Mr. Albany Hancock found twelve species of Clione on a single Tridacna. They are evidently not parasites, and I am not sure if their place is properly among messmates. The oyster, and more especially the Ostrea hippopus, lodges three or four different sorts in its shell. These Cliones possess siliceous spicules, by means of which they hollow out galleries in the substance of shells. Mr. Hancock has published a monograph of this genus, in which he recognizes twenty-four species collected from different shells, and two other species, which he refers to the genus Thoasa.

The cliones are real lodgers which lead us to the Saxicave, the Pholades, and the Teredines; they seek their lodging in rocks or in wood; these lead directly to the sea-urchins, which also hollow out lodgings in rocks, but without penetrating deeply. Professor Allman has just observed a very remarkable case of commensalism between a sponge and one of the tubulariæ. The crown of the tubularia is extended at the entrance of the canals of the sponge; and the association is so complete, that the Edinburgh professor imagined that he had before his eyes a true sponge with the arms of a tubularia.

In the lowest ranks of the animal scale, there are certain kinds of animalcules, which establish themselves on the bodies of obliging neighbours, and take advantage of their fins in order to swim at their expense. Thus we often find the bodies of certain crustaceans covered with a forest of vorticella and other infusoria. They cause themselves to be towed like cirrhipedes, but they do not change their toilet like them, so that it cannot be said that they put on the livery of servitude.

The kind of life led by several of these animalculæ is as yet little known.

Mons. Leydig has found in the stomach of the Hydatina Senta a messmate which much resembles an Euglena, and still more the Distigma tenax, Ehr.

CHAPTER III.

FIXED MESSMATES.

THE animals of which we have just spoken usually preserve their full and entire independence; from the time of their leaving the egg, till their complete development, they are subject to no other outward changes than such as belong to their class. If they sometimes renounce their liberty, it is only for a limited time; and they all preserve not only their peculiar appearance, but their organs intended for fishing or for locomotion. It is not thus with those which we are now about to consider; they are free in their youth, but as they draw near to puberty they make choice of a host, instal themselves within him, and completely lose their former appearance not only do they throw aside their oars and their pincers, but they cease sometimes to keep up any communication with the outer world, and even give up the most precious organs of animal life, not even excepting those of the senses; they are installed for life, and their fate is bound up with the host which gives them shelter. The number of these messmates is considerable.

We shall first allude to some crustaceans named Cirrhipedes by Lamarck. The metamorphoses which they have undergone since they left the egg have so