SCYLLIUM CANICULA 235 



its lateral corners produced into small lappets, the auriculae, and it 

 is situated in the dorsal part of the pericardial cavity. This chamber 

 communicates with the ventricle in the middle of its postero- ventral 

 border by a transverse slit guarded by a pair of auriculo- ventricular 

 valves. The ventricle is a very thick-walled muscular flask-shaped 

 structure on the ventral side of the pericardial cavity, and so it 

 forms the most obvious part of the heart when dissecting the fish 

 from the ventral side. The neck of the flask, as it were, is continued 

 forward as a thick muscular tube, the conus arteriosus, which passes 

 up to the ventral corner of the apex of the pericardial space, outside 

 which it is continued as a median artery, the ventral or cardiac 

 aorta. Inside the conus are two rows of semilunar valves, each 

 composed of three watch-pocket like flaps, the proximal row separat- 

 ing it from the ventricle. The conus is actually part of the heart, 

 being composed of the same cardiac muscle fibres that distinguish 

 the ventricle and taking part in the waves of contraction that pass 

 over the heart. The object of all the valves in the heart is to keep 

 the blood flowing in the same direction by preventing regurgitation 

 when fhe pressure behind it is released. 



It will be seen from the above description that the heart is in 

 reality a single tube, and its development shows that it originates 

 as the specialisation of a part of a median ventral vessel. The heart 

 of all vertebrates arises similarly as a single tube, which later becomes 

 bent upon itself in the form of an S. In Scyllium the heart is inte- 

 resting because it always remains in this primitive condition, and its 

 folded nature can be demonstrated very easily if it be removed from 

 the pericardium and cut in median longitudinal section. 



Arterial System. 



The vessels conveying the blood from the heart to the various 

 parts of the body constitute the arterial system. All the blood 

 leaves the conus by the one great trunk, the ventral aorta, which 

 runs forward below the hypobranchial elements giving off paired 

 branches, the afferent branchial arteries, to the gills. The ventral 

 aorta passes forward to the lower ends of the ceratohyal cartilages 

 to a spot immediately behind the thyroid gland, and there bifur- 

 cates into two branches, the innominate arteries, which turn directly 

 laterally. After a very short course each innominate divides into 

 two ; the first afferent branchial artery which runs along the hyoid 

 arch and supplies the first hemibranch, and the second afferent 

 branchial which runs along the first branchial arch and feeds the 

 posterior hemibranch of the first gill pouch and the anterior hemi- 

 branch of the second pouch, in other words the first complete gill. 

 The third afferent branchial artery arises about half-way back to the 



