ABSTRACT
The conus arteriosus is a muscular chamber interposed between the ventricle and ventral aorta in Elasmobranchii, Holocephalii and the more primitive families of bony fish. Since Duverney (1702) suggested that it functioned as an extra ventricle, it has been variously regarded as an elastic reservoir to minimize pressure fluctuations (Keith, 1924; Daniel, 1934), an accessory pumping chamber (Gegenbaur, 1866, Stöhr, 1876, Sudak, 1965), and a valvular structure to regulate the out-flow of blood from the ventricle (Home, 1823; Pettigrew, 1864; March, Ross & Lower, 1962). Both the conal pressure recordings of Sudak (1965) in Mustelus canis and the cinematographic analysis of March et al. (1962) in Triakis semifasciatus show that conus systole occurs prior to ventricular relaxation. However, our knowledge of the sequence of the valve movements is still largely conjectural. This paper reports an investigation of conal function in the Port Jackson shark, Heterodontus portusjacksoni. The anatomy of the conus will be briefly described, and a hypothesis of its mode of action will be out-lined. The evidence for this will then be presented, and the function of the conus discussed.
