In making observations upon any bivalve, such as the Mussel, it is extremely puzzling to know what is going on _inside_ the shell. Yet it is almost necessary to acquire this knowledge by means not always apparent, in order to satisfy one's mind relative to certain appearances, which we perceive going on externally. We have to form our judgment of things we do not see from those that are apparent--at all times a difficult task. But not often so tantalizing as in the case of an insignificant creature like the Mussel, who lives, moves, and works constantly before our eyes. I may add that it was not enough for me that I saw the spinning process frequently. It all seemed tolerably clear to my mind, but still I did not feel thoroughly satisfied. My desire was to peep into the shell, and find out where the last spun thread was situated; or, in other words, from what part of the trunk the new branch sprung. On examining various specimens of the byssus, this point was by no means apparent. Various means I adopted failed to secure me the requisite knowledge. At length I hit upon a plan, which, after no long time, I found opportunity to put in practice. My largest Mussel lifted up its testaceous canopy, put aside the fringed and fleshy veil that surrounded its edge, protruded its spinner to make sure the ground was secure, and then withdrew it again into the shell as usual. After the lapse of a second, the foot reappeared and was stretched out to an unusual length. No sooner was the end of the thread formed on the glass than immediately I firmly pressed the valves together, and held them in this position until I had gradually worked the Mussel up out of the vase, when I bound them close together by means of a piece of cord. I need not describe my manœuvres further; suffice it to say that the thread nearest to the base of the groove was found to be the one that was spun last. This, in my opinion, is invariably the case. I may mention that the above experiment also proved to my mind that the foot must be a most important vital organ of the Mytilus. At times, on placing an open Mussel in my tank as food for crabs or other animals, I have noted that if every other part were eaten, and the foot allowed to remain attached to the muscles of the bivalve, that member would after a lapse of several days show signs of--I do not say life--but sensation and retractile power. But when the foot is cut and otherwise injured, the animal dies quickly. In the experiment mentioned the valves were not kept closed for more than half an hour; yet when they were opened, vitality had evidently ceased within. This was the more singular when we remember that the Mytili will live for many days out of the water; the shells, of course, during the whole period being firmly closed. The Mussel, as already hinted, is very tenacious of life. I have kept specimens by accident for several days in the pocket of my coat, but found them quite well and lively when placed in sea-water.