176 



ZOOLOGY. 



at by a more complete doubling over of the heart on itself, 

 the dogfish being obviously nearer the primitive condition 

 of a straight tubular heart such as is found in embryonic 

 stages of all vertebrates. 



The truncus arteriosus consists, as in the frog, of two 



parts: the first, the 

 conus or pylangium^ 

 muscular, contractile, 

 and containing a series 

 of valves; the second, 

 the bulbus or synangium, 

 without valves. The 

 bulbus is often con- 

 veniently called the 

 ventral aorta. 



9. The Arteries. 

 From the bulbus there 

 branch, on either side, 

 four arterial trunks, the 

 first of which forks, so 

 that altogether there 

 are five afferent branchial 

 arteries taking blood to 

 be oxygenated in the 

 gills. The first of these 

 runs in the hyoid arch 

 (i.e. the arch between 

 spiracle and first gill- 

 cleft), the others in the 

 first four branchial 

 arches. Each sends 'off 

 branch arteries into all 



the gill-filaments, and finally dies out near the dorsal 

 end of the gill-arch. 



The blood from the gill-filaments is gathered into nine 

 vessels, one to each demibranch, or two to each afferent 

 artery except the first. These unite into four efferent 

 branchial arteries, as shown in fig. 92, and it will be seen 

 that the two demibranchs supplied by one of the afferent 



r/toc 



Fig. 91. DEVELOPMENT OF THE HEART OF 

 VERTEBBATA. 



