The study of the skeletal muscle of Limu/us was undertaken with two chief objects in view: (1) To attempt further to test my conclusion suggested by a series of earlier studies on the intercalated disks of vertebrate cardiac muscle, namely, that these disks are properly interpreted as "irreversible con traction bands." If this conception is correct it seems probable that something very similar to intercalated disks may be found in power fully acting skeletal muscle. Such structures have been repeatedly reported in vertebrate skeletal muscle, but the similarity to interca lated disks is still disputed. It must be emphasized, however, that even certain evidence of the presence of true intercalated disks in skeletal muscle is not crucial for the theory, but only confirmatory. Lack of such evidence can not invalidate the theory as applied to heart-muscle, nor does its possession necessarily prove it correct.
(2) To seek additional evidence in further refutation of the recently revived hypothesis that striped muscle can be interpreted in terms of "muscle-cells" and intercellular myofibril's).
A priori, the abdominal muscles which control the caudal spine of Limulus seemed to offer very favorable material for the search of the chief data desired. This surmise proved true beyond expectation as respects the second point. Moreover, the material yields very clear cut data also as to a number of other disputed details concerning muscle structure, chiefly the relation of the ground membrane to the sarcolemma, to the nuclear wall, and to the myofibrillie.
Concerning the first point no unequivocal evidence accrues. How ever, this study led to a reinvestigation of Limulus cardiac muscle with a different staining technic, and I can now report the presence of a very few, structurally of the simpler type, of intercalated disks. This obser vation would seem to warrant the generalization that cardiac muscle is characterized, at least as low as Limulta, by intercalated disks. The evidence concerning selachii and cyclostomes only remain uncer tain. On the other hand, the evidence may perhaps be regarded as confirming the belief of some zoologists that Linru/us is closely related to the more direct vertebrate ancestors. But similarity of histologic structure need not necessarily have phylogenetic significance; it may mean nothing more than the result of physiologic similarity. Ob viously much comparative microscopic investigation of cardiac muscles is desirable below teleosta.
Other points of considerable interest touch the very close funda mental structural similarity between the spine-muscle and the heart muscle of Limulus, and between the spine-muscle and an early stage of the development of striped muscle in teleosta.