The child has given a clue to the history of the old man. We will not examine these animals in all the details of their private life, and yet we are strongly tempted to confess to our readers some of the indiscreet acts of which we have been guilty, in watching them while changing their dress. Notwithstanding their shyness and their desire to escape observation during the moulting period, we have more than once made observations on them while quitting their garment which has become too small. The old tunic generally splits down the back, and falls off all in one piece as it gives the animal egress. The crustacean is extended quite soft and supple by the side of its rigid carapace. Of all the free crustacean messmates, one of the most interesting, though among the smallest of them, is a tiny crab, about as large as a young spider, which lives in mussels, and which has been often accused, though evidently wrongfully, as the cause of the indisposition so well known by those who are fond of this mollusc. Very many of them have been seen within the last few years, and yet accidents have been very few. The mussels themselves are guilty; they produce on some persons an injurious effect, through _idiosyncracy_. We have at least a word to serve as an explanation, and at present we must content ourselves with it. Under what conditions do those crabs, called by naturalists Pinnotheres, and which we do not find elsewhere, inhabit mussels? Are they parasites, pseudo-parasites, or messmates? It is not a taste for voyaging which tempts them, but the desire of having always a secure 16 retreat in every place. The pinnothere is a brigand who causes himself to be followed by the cavern which he inhabits, and which opens only at a well-known watchword. The association redounds to the advantage of both; the remains of food which the pinnothere abandons are seized upon by the mollusc. It is the rich man who instals himself in the dwelling of the poor, and causes him to participate in all the advantages of his position. The pinnotheres are, in our opinion, true messmates. They take their food in the same waters as their fellow-lodger, and the crumbs of the rapacious crabs are doubtless not lost in the mouth of the peaceful mussel. There is no doubt that these little plunderers are good lodgers, and if the mussels furnish them with an excellent hiding-place and a safe lodging, they themselves profit largely by the leavings of the feast which fall from their pincers. Little as they are, these crabs are well furnished with tackle, and advantageously placed to carry on their fishery in every season. Concealed in the bottom of their living dwelling-place (a den which the mussel transports at will) they choose admirably the moment and the place to rush out to the attack, and always fall on their enemy unawares. Some of these pinnotheres live in all seas, and inhabit a great number of bivalve molluscs. The northern seas contain