VASCULAR SYSTEM 



307 



contractile bulbus arteriosus usually lies close against the ventricle 

 (Fig. 246, c). . o 



The heart of Fishes contains venous blood only, which it forces 



Fia, 246. — DiAfiRAMMATic Longitudinal Section thkough the Hearts of 

 Vakious Fishes. (From Boas's Zoology.) A, Fish with well developed 

 conus anteriosns {e.g., Elasmobranoh) ; B, Amia ; G, a Teleost. In B and C 

 the sinus venosus and atrium are not indicated. 



a, atrium ; h, bulbus arteriosus ; >■, conus arteriosus ; /;, valves ; 

 t, ventral aorta ; v, ventricle. 



smus venosus : 



through the afferent branchial arteries (Figs. 243, B, 245, c, and 264) 

 into the capillaries of the gills, where it becomes oxygenated, to 

 pass thence into the efferent branchial arteries, and so into the 

 aortic roots. 



Dipnoi. — In the Dipnoi, as in Fishes proper, the heart lies 

 far forwards, near the head. In correspondence with the double 

 mode of respiration, by lungs as well as by gills, it reaches a 

 stage of development mid-way between that seen in Elasmo- 

 branchs and in Amphibians (Figs. 247 and 248). The atrium 

 becomes divided into a left and a right chamber by a septum, as 

 does also the ventricle to some extent, owing to the presence of 

 a cushion composed of muscular fibres and connective-tissue (Fig. 

 247) situated between the atrium and ventricle, and extending 

 into both of these chambers : this acts as a valve, ordinary atrio- 

 ventricular valves being absent. The sinus venosus, from the 

 Dipnoi onwards, opens into the right atrium. 



The conus arteriosus is twisted spirally on itself (Fig. 248) : in 

 Ceratodus it is provided with eight transverse rows of valves, and 



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