170 



THE ELASMOBRANCH FISHES 



CIRCULATORY SYSTEM OF ELASMOBRANCHS IN GENERAL 



The circulatory apparatus in the Elasmobranchs in general consists of four 

 structures, all formed first as simple tubes. These structures are (1) a rela- 

 tively simple two-roomed heart, the walls of a part of which have become thick- 

 ened for pumping; (2) arteries, which bear blood from the heart and from the 

 gills; ^3) a series of terminal thin-walled capillaries which connect the arteries 



with (4) the veins. The veins in 

 Elasmobranchs are relatively large 

 and return the blood to the heart. 

 As is true for vertebrates in gen- 

 eral, the arteries and veins are dis- 

 tinguished by two or three well de- 

 fined characteristics. Both are made 

 up of layers from the inside out as 

 follows : a thin lining, the intima, 

 around which is the muscularis or 

 muscular layer. Surrounding the 

 muscular layer is the serosa. But in 

 the artery the muscular layer is 

 much thicker than it is in the vein. It 

 is this layer in the artery which 

 keeps the blood under more constant 

 pressure and forces it through the 

 capillaries and veins back to the heart. A second distinction between the two 

 is that the veins possess valves. These valves are numerous and are especially 

 large at the ends where the vessels empty. They are so arranged that blood 

 can pass freely towards the heart, but its passage in the opposite direction 

 is precluded. 



The blood stream in the Elasmobranchs, as in other vertebrates, is made up 

 of a relatively large amount of plasma in which is contained a small amount of 

 serum. The erythrocytes or red corpuscles (fig. 158) contain only a small 

 amount of haemoglobin or red coloring matter and are nucleated cells much 

 greater in diameter than are the red corpuscles of man. The white corpuscles 

 are also nucleated and may be filled with granules. 



Fig. 158. Ked blood corpuscles of Sqiialns 

 sucMii. (M. C. Williamson, orig.) 



He.^t 



The heart in the Elasmobranchs, as was seen in Heptanch us, does not undergo 

 the differentiation characteristic of the more complex heart of higher animals. 

 As a usual thing, it is composed of a thin-walled auricle (atrium) (a«., fig. 159) 

 and a thick-walled ventricle {vn.). The auricle receives the non-oxygenated 

 blood from the sinus venosus {s.v.) and the the ventricle sends it forward 

 through the conus (truncus) arteriosus (c.o.). 



