506 DIPNOI [CH. 



shall term them the lungs, by two pulmonary veins. The conus 

 of the heart is spirally twisted and contains several transverse rows 

 of pocket valves. In Ceratodus one valve in each row is enlarged 

 so as to touch one in the next row, and in this way an obliquely 

 longitudinal valve is foreshadowed, which becomes a definite struc- 

 ture in Lepidosiren and Protopterus. There is no ventral aorta ; 

 the afferent branchial arteries, of which there are four or five pairs, 

 arise in two groups one group from the distal end of the conus, 

 whilst the hinder group, though becoming distinct at the end of 

 the conus, really arises from its dorsal surface some distance back. 

 The longitudinal valve is inserted in such a way that when the free 

 end is pressed against the wall of the conus the valve covers the 

 openings of this hinder group of afferents. The efferent arteries 

 are five in number on each side, each made up of a pair of vessels, 

 one of which drains the gill on the front of the arch and one the 

 gill on the back. The spiracle is lost, and the first cleft has a gill 

 only on its anterior border. In Ceratodus this gill is a "pseudo- 

 branch," supplied by a branch from the first efferent running 

 round the ventral edge of the cleft. The vessel which carries the 

 blood from this gill, as in Teleostei, is a carotid artery (Fig. 247). 

 In Protopterus, however, this hyoidean gill receives an afferent 

 vessel from the ventral aorta, and its efferent joins the dorsal aorta. 

 In this genus and in Lepidosiren there are no gills on the hinder 

 wall of the hyoidean cleft, on either wall of the next cleft, and 

 on the front wall of the third cleft, corresponding afferent and 

 efferent vessels form one continuous "arterial arch." The arteries 

 supplying the lungs, i.e. the pulmonary arteries, arise from a stem 

 formed by the junction of the last two or three efferents ; the first 

 two efferents on each side form a distinct stem opening separately 

 into the dorsal aorta. 



During the contraction of the heart the oblique longitudinal 

 valve is pressed down so as to close the entrance to the last two 

 afferent arteries on each side. The valve is so situated that the 

 blood which pours into the atrium from the sinus venosus is directed 

 by it to the openings of these last two afferents, and so the bulk of 

 it is directed to the lung, whilst the blood from the left auricle 

 which enters the other side of the ventricle is shut off by the valve 

 from entering the last two afferents, and goes forward to enter the 

 first two afferents which spring from the anterior part of the conus, 

 and so through the carotids to the head that passing through 

 the hyoidean gill receiving a further purification. A very similar 



