42 ALLEN 



connective tissue layer (Text-figs, i and 2, and PL VI, fig. 3^, 

 C.T.), which is continuous with the corresponding layer of the 

 auricle and where such union takes place, the layer becomes 

 much thicker (see Text-fig. i). The trabeculce carnce of the 

 ventricle (PL VI, fig. 40; T.C.V.) resemble the trabeculse of 

 the auricle close to the outer connective tissue layer, the blood 

 cavities being very small. In addition to the 2 layers of the 

 auricle, the ventricle has a thick musctilar layer (PL VI, fig. 

 40 ; M.L.) between the trabeculas carnge and the outer connec- 

 tive tissue layer, containing no blood spaces. The central cav- 

 ity of the ventricle (PL VI, figs. 39 and 40 ; C.C.V.), which 

 runs close to dorsal wall, is much smaller than the central cav- 

 ity of the auricle. The anterior end of this cavity is continuous 

 with the posterior end of the bulbus arteriosus, which represents 

 the well developed conns arteriosus (PL VI, fig. 40 ; C. Art.) of 

 the Elasmobranchs and Ganoids. The entrance into the conus 

 arteriosus is guarded by 2 semi-lunar valves (Text-fig. 2 and 

 PL VI, fig. 40; S.V.), dextrad and sinistrad in position and 

 having their inner margins free. These valves are similar to 

 the auriculo-ventricular valves, except that each valve has 2 

 cephalic processes, which continue along the dorsal and ventral 

 walls of the bulbus arteriosus. A reverse current, caused by a 

 retraction of the elastic walls of the bulbus, would entirely close 

 these valves, allowing no blood to return to the ventricle. 



Bulbus Arteriosus (Pis. I, II and VI, figs. 1,12 and 40, and 

 Text-fig. 2 ; B. Art.). — After leaving the ventricle the bulbus 

 rapidly increases in diameter and then gradually tapers down 

 into the ventral aorta or branchial artery (Pis. I. and II, figs. 

 I and 12 ; V.Ao.), which gives off the paired afferent branch- 

 ial vessels to the gills. The walls of the bulbus are quite thick 

 and the internal layers are thrown into longitudinal ridges or 

 folds (PL VI, fig. 40 ; L.F.). The bulbus is richly supplied 

 with blood vessels, which will be described later on. 



Microscopical Structure of the Heart (Text-figs, i and 2). — 

 A transverse section through one of the auriculo-ventricular 

 valves (Text-fig. i) shows us that the outer connective tissue 

 layer is greatly thickened in the dorsal portion of the auricle and 

 at the union with the same layer of the ventricle. As in other 

 vertebrates the muscle fibers are striated and run in all direc- 



