CHAPTER XI 

 THE CIRCULATORY SYSTEM AND BLOOD 



Page 



General characteristics 239 



The pericardium 239 



The heart 240 



Physiology of the heart 242 



Automatism of heart beat 242 



The pacemalier system.. 245 



Methods of study of heart beat 247 



Frequency of heat ___ 248 



Extracardiac regulation 250 



Effects of mineral salts and drugs 251 



Blood vessels _. 253 



The arterial system 253 



The venous system 254 



The accessory heart 258 



The blood 259 



Color of blood 261 



The hyaline cells 261 



The granular cells 262 



Specific gravity of blood 265 



Serology. 265 



Bibliography 266 



A heart, arteries, veins, and open sinuses form 

 the circuhitory system of oysters and other bi- 

 valves. The sinuses, or lacunae, are irregular 

 spaces of varying size in the tissues and have no 

 walls of their own other than the surrounding 

 connective tissue. They are interposed between 

 small arteries and veins and function in place of 

 the capillaries of vertebrates. Blood cells are not 

 confined to the vessels; they wander throughout 

 the tissues, aggregating in the sinuses. A large 

 number of them accumulate on the surface of the 

 mantle and gills and are discarded. Diapedesis, 

 i.e., slow bleeding tlirough the surface of the body, 

 is a continuous and normal process which is ac- 

 celerated by adverse conditions, by injuries to the 

 tissues, and by removal of an oyster from its sliell. 



The open sinuses within the circulatory system 

 present a mechanical puzzle. It is difficult to 

 visualize how the pressure of the systolic contrac- 

 tion forces the blood to leave the open spaces and 

 enter the venal system, which has no valves, go 

 through a complex net of branchial vessels and 

 finally enter the heart. To a great e.xtent the 

 mechanical deficiency of the circulatory system is 

 compensated by the pulsating vessels of the mantle 

 and by the contractions of two accessory hearts on 

 the walls of the cloacal chamber. The pulsations 



FISHERY bulletin: VOLUME 64, CHAPTER XI 



of these organs are independent of the beating of 

 the principal heart, and their primary function is 

 to oscillate the blood within the pallial sinuses. 



THE PERICARDIUM 



The heart is located in the pericardium, a thin- 

 walled chamber between the visceral mass and the 

 adductor muscle (fig. 71). In a live oyster the 

 location of the heart is indicated by the throbbing 

 of the wall of the pericardium on the left side. 

 Here the pericardium wall lies directly under the 

 shell. On the right side the promyal chamber 

 extends down over the heart region and tlie mantle 

 separates the pericardium wall from the shell. 



The cavity in which the heart is lodged is slightly 

 asymmetrical ; on the right side it extends farther 

 along the anterior part of the adductor muscle 

 than on the left. The pericardium is large enough 

 to accommodate the heart and to retain a supply 

 of the fluid in which the heart is bathed. The 

 volume of the pericardium can be measui-ed by 

 the following metliod. A solution of plastic or a 

 thin mixture of plaster of paris is poured into the 

 pericardium from which the heart has been re- 

 moved; after the material has set, the plaster 

 molds are waterproofed by immersing them in a 

 hot mixture of beeswax, rosin, and turpentine. 

 The volumes are measured by displacement. In 

 an adult Crassostrea virginica about 12 to 14 cm. 

 in height, the capacity of the pericardium varied 

 from 2.4 to 2.7 ml. ; approximately the same 

 \'olunie of blood and pericardial fluid could be 

 withdrawn from the cavity by hypodermic syringe. 



Two reno-pericardial canals open on the right 

 and left side of the ventro-posterior wall of the 

 pericardium and provide direct communication 

 with the excretory system (see: ch. XII). The 

 wall of the pericardium is formed of connective 

 tissue similar to that in the mantle; the tissue is 

 well supplied with blood vessels, blood sinuses 

 (figs. 211 and 212), and branches of the cardiac 

 nerve (fig. 213). The epithelium lining of the side 



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