The anatomy and histology of the heart of Limulus Polyphemus have been described by Patten and Redenbaugh, Milne-Edwards and Walter Meek, but they are not in complete agreement. In the case of Branchio cardiac canal, for instance, Milne-Edwards reported six pairs of veins but Patten and Redenbaugh five pairs. There is little recorded work on the anatomy and histology of the heart of Tachypleus tridentatus in Japan, and although Dr. Nukada and Dr. Hoshino made brief anatomical and histological description for the purpose of Physiological study, they also disagree on some points. The former maintains and the later denies the presence of Volhof and Spezifische Muskelfasern. On account of these discrepancies, I made certain studies in comparison with previous results and some researches into new ground. Most of the materials were fixed in formalin and after subsequent treatment in the usual manner, the specimens were embedded in paraffin, trimmed and cut in transverse serial sections and longitudinal sections with thicknesses of 5.0, 25.0, and 50.0 microns. The sections were stained with Eosin-Hämatoxylin and Nissl's method was used.
A model was prepared according to Born-Peter's method of wax-plate reconstruction. From the results of my experiments, the following summary and conclusions were made: 1) The heart of Tachypleus tridentatus is a long tubular organ consisting of one ventricle, situated longitudinally on the lower part of median line of the carapace. It lies in a large pericardial sinus and is pupported by three holding tissues, Ligamentum suspensoria cordis superior, Alary muscle, and Veno-pericardiac muscle. 2) Around the heart is a large pericardial sinus, enclosed by a pericardium. Pericardium is a thin menbrane consisting of connective tissue fibres. There are eight pairs of Ostia on the dorsal wall of the heart, and they are the doors by which the blood in the pericardial sinus enters the heart. 3) There are eleven arteries leading off from the heart; one pair of Aortic arches and Arteria frontalis from the frontal end of the heart, and four pairs of Arteriae laterales arise from the lateral wall of the heart. Each artery has a valve. The aortic valve, much like a vertebrate Semilunar valve, lies upon the dorsal wall of the heart at the base of the Aortic arches. The Arteriae laterales are supplied with paired Semilunar valves at their points of origin from the heart. Each valve consists of connective tissues. 4) Pericardial sinus has veins, namely Branchio cardiac canal. I found five pairs of Branchio cardiac canal. I could not recognize six-pairs as described by Milne-Edwards. 5) The structure of the cardiac walls is composed of three layers. The outer most layer consists of the longitudinal elastic conective tissue fibres. The middle layer is the Basement membrane consisting of annular connective tissue fibres. The inner most layer is the circular muscular layer consisting of cross-striated muscular fibres. No membrane limiting the lumen, corresponding to the Endocardium of the mamalian heart is present. The cross-striated muscular fibres of Tachypleus tridentatus, resemble the mammalian heart, branching and anastomosing with each other, so form a syncytium. Also the individual muscular fibres are much like those of nammals, having nuclei surrounded by sarcoplasm and the fibrils make numerous bundles or Cohnheim's field. I think that the muscular fibres have no sarcolemma. 6) The author could not recognize the existence of Volhof and Spezifische Muskelfasern which were Dr. Nukada. 7) An median Cardiac nerve and one pair of Lateral cardiac nerves lie longitudinally on the heart. The branches of these two kinds of cardiac nerves anastomosing with each other, thus make the Cardiac nerve plexus. Median Cardiac nerve communicates with the centre by the Segmental cardiac nerve, Haemal nerve and Ventral cord. The structure of the nerve fibre consists of neurolemma and axis cylinder. There is no medullary sheath. The median cardiac nerve fibers possess the ganglion cells.