CHAPTEE X. 



ORGANS OF CIRCULATION. 



The Blood-corpuscles of fishes are, with one exception, of 

 an elliptic shape ; this exception is Petromyzon, which pos- 

 sesses circular, flat, or slightly biconvex blood-corpuscles. 

 They vary much in size ; they are smallest in Teleosteans and 

 Cyclostomes, those of Acerina cernua measuring ^iVr of ^^^ 

 inch in their longitudinal, and ao^oo in their transverse 

 diameter. As far as it is known at present the Salnwnidce 

 have the largest blood-corpuscles among Teleosteans, those of 

 the salmon measuring j-o-r ^7 ttbtt ^^-i approaching those of 

 the Sturgeon. Those of the Chondropterygians are still larger ; 

 and finally, Z&pidosiren has blood-corpuscles not much smaller 

 than those of Perennibranchiates, viz. — -g-f g- by ^y in. 

 Branchiostoma is the only fish which does not possess red 

 blood-corpuscles. 



[See G. GuUiver, "Proc. Zool. Soc," 1862, p. 91 ; and 1870, p. 844 ; and 

 1872, p. 833.] 



Fishes, in common with the other Vertebrates, are pro- 

 vided with a complete circulation for the body, with another 

 equally complete for the organs of respiration, and with a 

 particular abdominal circulation, terminating at the liver by 

 means of the vena porice ; but their peculiar character con- 

 sists in this, that the branchial circulation alone is provided 

 at its base with a muscular apparatus or heart, corresponding 

 to the right half of the heart of Mammalia and Birds. 



The Heart is situated between the branchial and abdo- 

 minal cavities, between the two halves of the scapulary arch. 



