A friend, on one occasion having procured for me, among other objects, a Common Limpet, I placed this mollusc in my aquarium, and soon had the pleasure of watching it affix its broad foot to the surface of the glass. After a while, on the Limpet slightly raising its canopy, I was surprised to observe a little Shore-Crab peer out from between the foot and shell. On suddenly ejecting the intruder by means of a small brush, he speedily hid himself from view among the surrounding pebbles. A few hours after, on again approaching the tank to view the Patella (which was easily identified, from the fact of an immense colony of Mussels being settled on its back), I found to my great astonishment that the crab had re-seated himself in his old position. I often repeated the sweeping operation, but without success, for the little rascal had become artful, and was not inclined to be driven forth a second time by a _coup de main_. I touched the Limpet frequently and saw it glue itself, as usual, to the glass; but, singular to state, the creature always left a larger space between its foot and the circumference of the shell on the side at which the crab was seated, than on the opposite one, seemingly from a wish to accommodate its crustaceous friend. This space, moreover, let me observe, was larger than was absolutely necessary, for, as the shell was not air-tight, I was enabled to thrust my camel-hair pencil teazingly upon the crab, and was much amused to watch him clutch at the intruding object, and, at times, move about with it in his grasp, thus proving that he was by no means uncomfortably 'cabin'd, cribb'd, confin'd.' For a whole week the crab remained in his favorite lodgings, and only resigned occupancy thereof when his friend gave up the shell--and died. There is a certain species of crab, _Pinnotheres pisum_, or common Pea-Crab, frequently found in _Mytilus edulis_, the Oyster, and the Common Cockle. Indeed, one gentleman states, that on his examining, on two occasions, a large number of specimens of the _Cardium edule_, he found that nine out of every ten cockles contained a crab. Still, in no other instance than the one my own experience furnishes, have I ever heard of the Shore-Crab, or, indeed, of any other crustacean, becoming the guest of _Patella_.