For the successful prosecution of the histological inquiry which forms the subject of the present communication, it is essential that the tissue to be investigated should be studied whilst still in the living condition, inasmuch as marked changes ensue very speedily after the death of the muscle, and still more speedily on the addition of reagents, even the so-called indifferent fluids, such as serum or ½ per cent. solution of common salt, being in this case inadmissible. The description, therefore, I have now to give is founded entirely on an examination of the living tissue. I. Appearance of Living Muscle in the state of rest If we cut off a limb from one of the common large Water-beetles ( Dytiscus marginalis ), remove a portion of muscle from the upper part, quickly separating the fibres somewhat from one another, by means of needles, on a glass slip, cover it without addition, and examine the preparation so obtained with the aid of a very powerful immersion-objective (such as the No. 11 of Hartnack or the No. 3 of Zeiss), we find numerous muscular fibres presenting an appearance similar to that represented in Plate XXXIII. fig. 1.