XII VASCULAR SYSTEM 329 
Fishes the vestigial conus is sueceeded by a non-contractile, bulb- 
like dilatation, or bulbus aortae, of the root of the ventral aorta. 
In only a single Teleost, viz. Albula, 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 
1 Stannius, Handb. d. Anat. d. Wirbelth. Berlin, ii. 1854, p. 235; Boas, Morphol. 
Jahrb. vi. 1880, p. 527. 
2 Boas, Morphol. Jahrb. vi. 1880, p. 321. 3 Ibid. op. cit. 
4 J. Miiller, Vergl. Anat. d. Myxinoiden, Pt. iii. (1839) Berlin 1841 p. 179, 
