. Bulletin of the British Museum (Natural History). •OBSERVATIONS ON THE HEART IN THE FAMILY TRIONYCHIDAE 97 antero-medially and ventrally towards the posterior end of the free edge of the inter- ventricular septum. Here the fibres aggregate to form a fairly thick band which is further enlarged by additional bands which run in a dorso-ventral direction and join the main bulk at its left side. A secondary septum is thus formed which extends between the dorsal, ventral and posterior walls. From the edge of the secondary septum the fibres spread fanwise as they proceed anteriorly in the ventral w
. Bulletin of the British Museum (Natural History). •OBSERVATIONS ON THE HEART IN THE FAMILY TRIONYCHIDAE 97 antero-medially and ventrally towards the posterior end of the free edge of the inter- ventricular septum. Here the fibres aggregate to form a fairly thick band which is further enlarged by additional bands which run in a dorso-ventral direction and join the main bulk at its left side. A secondary septum is thus formed which extends between the dorsal, ventral and posterior walls. From the edge of the secondary septum the fibres spread fanwise as they proceed anteriorly in the ventral wall of the ventricle to terminate in the anterior border of its base. The spreading out of the fibres reduces their depth and leaves a clear anterior passage between the left and right sides of the ventricular cavity. arc. sys. arc. Fig. 13. C. oligotylis. Posterior view of a dorso-ventral section in the anterior part of the ventricle to show the openings of the arterial arches in the ventricular cavities. cav. d., cavum dorsale ; cav. p., cavum pulmonale ; intv. sep., inter-ventricular septum ; pul. arc. c. op., opening of common pulmonary arch ; sys. arc. 1. op., opening of left systemic arch ; sys. arc. r. op., opening of right systemic arch. The secondary septum in T. triunguis is less prominent and is nearer to the posterior wall, so that the division of the cavum magnum posteriorly into two cavities is not as sharp and clear as in C. oligotyhts. The position of the openings of the arterial arches (Text-fig. 13) shows no difference from T. triungtiis. 5. Heart Beat The two auricles contract in the same way as in T. triunguis driving the blood to the ventricle, which enlarges considerably. Ventricular systole follows ; the left side of the ventricle contracts first and the right side immediately afterwards, the whole ventricle remaining in the contracted condition for an interval during which its wall changes colour from dark red to pale pink as b
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