202 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



more (in Lepidosteus eight or nine) longitudinal rows 

 of pocket-like valves ; in Lepidosteus there are four 



well-developed and four smaller 

 valves in each of the nine planes, 

 so that were they all complete there 

 would be as many as seventy-two. 



Among the Dipnoi, Ceratoclus 

 has one or more rows of well- 

 developed pocket valves, but the 

 fact that the number is inconstant 

 shows that a change is impending ; 

 such a change is found in Protop- 

 terus, where the valves are few in 

 number and minute in size, while 

 their place is taken by a longitu- 

 dinal fold, which extends down the 

 greater part of the cone, and very 

 possibly owes its origin to a fusion 

 of a row of valves. By means of 

 the valve the cone is divided into a 

 right and a left half, and the blood 

 that has just returned from the 

 body is now carried to the third 

 and fourth arches, the latter of 

 which gives off a large pulmonary 

 artery, or vessel which goes direct 

 to the lungs. 



The essential parts of this ar- 



Fig. 87. Diagram of 

 the Arterial Circu- 

 lation in Fishes. 

 (AfterWiedersheim.) 



rangement are seen among some of 



the Amphibia ; but, as may be sup- 

 posed from what has already been 

 said of the arrangement of the ventricle in the lower 

 Reptilia, no functionally independent arterial cone is 

 to be observed in them ; nor is it seen in the adults of 

 the higher Vertebrates, though even there it is at first 

 a distinct part of the heart, and is undivided both 

 within and without. 



