VASCULAR SYSTEM 329 



Fishes the vestigial conus is succeeded by a non-contractile, bulb- 

 like dilatation, or bulbus aortae, of the root of the ventral aorta. 

 In only a single Teleost, viz. Alhula, one of the Albulidae, is the 

 vestigial conus muscular, and at the same time provided with 

 two rows of valves.^ In the Cyclostomata there is a bulbus 

 with a single row of two valves, but no true conus. 



In the Dipnoi (e.g. Protopterus) the heart, like the rest of 

 the vascular system, exhibits certain interesting resemblances to 

 the Amphibian heart. In addition to a more or less complete 

 interauricular septum separating right and left auricles, there is 

 a median longitudinal ridge, partly muscular and partly fibrous, 

 which incompletely subdivides the cavity of the ventricle. The 

 spirally-twisted conus arteriosus is furnished with several trans- 

 verse rows of valves, certain of which coalesce longitudinally to 

 form a complete septum dividing the cavity of the conus into 

 two distinct lateral channels : with this septum there coalesces 

 another septum, which cuts off the origins of the anterior two pairs 

 from the remaining afferent branchial arteries. The formation of 

 these septa has the physiological effect of subdividing the series of 

 cardiac cavities into two parallel channels, of which one has its 

 origin behind in the sinus venosus and transmits venous blood 

 to the posterior afferent branchial vessels; while the other, com- 

 mencing with the left auricle, conveys arterial blood to the first 

 two pairs of afferent branchial arteries.^ In Neoceratodus, however, 

 the longitudinal septum in the conus is incomplete, and hence 

 the blood which is sent to the anterior afferent vessels is mixed.^ 

 The Arterial System. — The ventral aorta is a median artery 

 situated beneath the floor of the pharynx, and having its origin, 

 behind, either directly from the ventricle or from the conus 

 arteriosus. 



In the. Cyclostomata* (e.g. Petromyzon) the ventral aorta 

 (Fig. 194) is continued forwards from the heart as a single 

 vessel to the fourth pair of gill-sacs, where it divides into right 

 and left branches which extend as far as the anterior walls of 

 the first pair of gill-sacs. Eight pairs of afferent branchial 

 arteries arise from the ventral aorta and its two branches, of 



^ Stannius, Handb. d. Anat. d. Wirhelth. Berlin, ii. 1854, p. 235 ; Boas, Morphol. 

 Jahrb. vi. 1880, p. 527. 



2 Boas, Morphol. Jahrb. vi. 1880, p. 321. ^ Ibid. op. dt. 



* J. Miiller, Vergl. Anat. d. Myxinoiden, Pt. iii. (1839) Berlin 1841 p. 179. 



