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Show 254 MR. F. E. BEDDARD ON THE [May 1, quotation from his paper will show that he probably refers to a structure identical with that described in the present note (loc. cit. p. 236) :- " Inde'pendamment de ces deux faisceaux musculaires que Ton retrouve avec la meme disposition chez tous les oiseaux, j'ai observe chez les Eudyptes au niveau de l'ongle forme en dehors par le diaphragme thoraco-abdominale etla portion inferieure du diaphragme thoracique, un faisceau musculaire a fibres pales et divergentes. Ces fibres sont dirigees suivant le contour qu'affecte en leur point d'existence la cavite abdominale. Elles sont assez courtes et se terminent toutes par un sommet aponeurotique. Je designerai ce muscle par l'appeliation de muscle diaphragmatique transverse." It apipears therefore that the Puffins as well as the Penyuins and Ducks are to be distinguished from many other birds by the fact that the oblique septum is partially covered by a layer of muscular fibres. But this layer of muscular fibres is by no means equally developed in all the three groups cf birds. It is best developed in the Puffin and in the Penguins ; it appears to be very feebly developed in the Duck. Prof. Huxley gives no particular description of it in the Duck, but his figure ' shows that the layer of muscular fibres is very limited in extent and does not reach nearly as far as the sternum. It is attached to the dorsal middle line of the body and only covers tbe oblique septum for a very short way. I have found in the Toucan (Rhamphastos ariel) a perfectly similar patch of muscle occupying an identical position; the muscular fibres in this case also were obviously unstriated. There is another important difference between the muscular layer of the oblique septum in the Duck and that in the Puffin. The fibres are in the Duck (according to Prof. Huxleg) and in the Toucan unstriated; in the Puffin they are distinctly striated: unfortunately neglected to observe whether this was also the case iu tbe Penguins, and Filhol makes no mention of the point. It does not, however, as it appears to me, affect the question of the homology of the muscular layer covering the oblique septum in these three types to learn that the fibres are striate in the one and unstriate iu the other. The muscular fibres of the alimentary tract are commonly said to be striated in the Tench, while they are unstriated in other fish. It is unnecessary to insist upon the homology of the muscular layer in the two cases. Mr. C. F. Marshall, in a paper upon the histology of muscle 2, points out that striation in the fibres of muscle-bundles appears to be associated with greater activity on the part of the muscle. The muscles, for example, of an Echinus are for the most part unstriated ; those muscles which move the valves of the pedicellarise are striated, as was shown by Mr. Geddes and myself, and more recently by Hamann. The pedicellarise are undoubtedly the most actively moving organs of the Echinus ; and the nature of their muscles (striate) supports the views of Mr. Marshall. 1 Loc. cit. p. 565, fig. 2, m. 2 Quart. Journ. Micr. Sci., Aug. 1887. |
