Memoirs: The Conus Arteriosus in Fishes

Abstract

1. The term ‘conus arteriosus’ is used to define the whole of the headward portion of the heart in fishes which intervenes between the ventricle and the anterior boundary of the pericardiac space. 2. A brief description of the conus arteriosus in a number of different forms is given and attention is directed particularly (a) to the musculature of the conus wall, (b) to the number of endocardiac ridges or valves which it contains, and (c) to the position of these ridges in relation to the headward boundary of the pericardiac cavity. The latter is taken as a fixed point morphologically and therefore as giving true indications of the positions of the ridges (right, dorsal, left, and ventral). Thus, in Dipnoi, the conspicuous longitudinal ridge or fold which forms the conus septum is morphologically right in position. The hearts examined include those of Acanthias, Raia, Scyllium, and Pristiurus , Polypterus and Lepidosteus, Amia, the Dipnoi, and various Teleostei. Among the latter are Megalops, Gymnarchus, and Symbranchus. 3. The tendency to reduce the contractile function of the conus and to restrict its valves to the ventricular end is noted even within the Elasmobranchii. The process has advanced so far in Teleostei that the entrance to the lumen of the conus is protected by only one pair of valves as a rule, and the conus has lost its myocardiac covering. Megalops is one of a small number of exceptional Teleostei in which the endocardiac valves are more numerous. 4. The number and position of endocardiac valves in Cyclostomata is similar to their number and position in Teleostei. 5. Discussion is confined (1) to the primitive number of endocardiac ridges ; (2) to the evolutionary significance of the conus arteriosus in fishes ; (3) to the function of the conus arteriosus in Teleostei.