THE DOGFISH 359 



by the shell, the young dogfish develops slowly at the 

 expense of the yolk, which comes to be contained in a sac 

 attached to its belly. At one stage long, vascular threads 

 project from the gill clefts of the little fish. These are the 

 so-called external gills, but they are covered with endoderm 

 and thus differ from the true external gills of the tadpole. 

 The heart of a dogfish lies in the pericardium between 



the hinder gill-clefts of the right and left sides. 

 Heart. VeSS8lS : ^ * s a median structure with muscular walls, and 



consists essentially of an irregular tube, bent 

 like an S upwards and downwards (Fig. 251) and composed 

 of four successive chambers. The hindermost chamber is 

 the thin-walled sinus venosus, which is triangular as seen 

 from below, and lies with its base against the hinder wall 

 of the pericardium. In front of it comes the thicker walled 

 auricle or atrium. This is also triangular, with its apex 

 forwards, and has its hinder angles widened into pouches, 

 but is not divided into two chambers like, that of the frog. 

 The S then curves downwards, as the very thick-walled, 

 conical ventricle, which lies below and somewhat behind 

 the auricle. From it the narrow conus arteriosus passes 

 forwards through the front wall of the pericardium to 

 become the ventral aorta, which is merely the foremost part 

 of the single vessel whose thickening and twisting produces 

 the heart behind. Thus the heart, or contractile blood- 

 vessel, of the dogfish, like that of the frog and all other 

 vertebrate animals, is ventral in position, whereas the 

 principal contractile vessel of an invertebrate is generally 

 dorsal. The heart contracts from behind forwards, and 

 drives blood into the ventral aorta, reflux being prevented 

 by a valve at the opening of the sinus into the auricle, 

 another at the auriculo-ventricular opening, and two rows 

 of semilunar or watch-pocket valves in the conus. 



The ventral aorta lies in the middle of the throat, below 



the pharynx and between the gill clefts, giving 



off afferent branchial arteries to the fourth, third, 

 and second branchial arches, and ending by dividing into 

 two vessels, each of which again forks to supply the first 

 branchial and hyoid arches of its side. There are thus five 

 afferent branchial arteries. These, together with the ventral 

 aorta, form the ventral arterial system. The thyroid gland, 



