1. In all muscles which have to perform rapid and frequent movements, a certain portion of the muscle is differentiated to perform the function of contraction, and this portion takes on the form of a very regular and highly modified intracellular network. 2. This network, by its regular arrangement, gives rise to certain optical effects which cause the peculiar appearances of striped muscle. 3. The contraction of the striped muscle-fibre is probably caused by the active contraction of the longitudinal fibrils of the intracellular network; the transverse networks appear to be passively elastic, aud by their elastic rebound cause the muscle to rapidly resume its relaxed condition when the longitudinal fibrils have ceased to contract; they are possibly also paths for the nervous impulse. 4. In some cases where muscle has been hitherto described as striped, but gives no appearance of the network or treatment with the gold and other methods, the apparent striation is due to optical effects caused by a corrugated outline in the fibre. 5. In muscles which do not perform rapid movements, but whose contraction is comparatively slow and peristaltic in nature, this peculiar network is not developed. In most if not all of the invertebrate unstriped muscle there does not appear to be an intracellular network present in any form, but in the vertebrate unstriped muscle a network is present in the form of longitudinal fibrils only; this possibly represents a form of network intermediate between the typical irregular intracellular network of other cells and the highly modified network of striped muscle. 6. The cardiac muscle-cells contain a network similar to that of ordinary striped muscle. The investigations connected with this paper were partly carried on in the laboratories of the Owens College and partly at the Scottish Marine Station at Granton. I must here express my thanks to my brother, Professor Milnes Marshall, for his kindness in revising the paper, for much advice in its production, and for obtaining the literature of the subject; all the controversial points were discussed with him and the preparations submitted to his examination. My thanks are also due to Dr. Klein for kindly showing me his preparations and for examining several of my own. I must also thank Mr. J. T. Cunningham for the use of the Scottish Marine Station, and for obtaining several of the animals.