p<t/, 



ac h 



Figure 236. — The position of the accessory heart on the 

 left of the cloacal wall of C. virginica. The epibranchial 

 chamber was dissected, and the demibranchs of the 

 right and left side pulled apart to expose the ventral 

 side of the adductor muscle. The oyster was fully 

 narcotized. The accessory hearts on both sides 

 were fully expanded (only the part of the right accessory 

 heart is shown), a. — anus; ac.h. — accessory heart 

 on the left side; ad.m. — adductor muscle; m. — mantle; 

 pal. or. — pallial organ; r. — rectum. 



wall of the cloaca to the pallio-branchial junction 

 (p.br.j.). 



Under slight mechanical stimulation the delicate 

 wall of the accessory heart collapses and the 

 structure becomes invisible. This explains why 

 the earlier investigators did not recognize it as an 

 active organ and mistook it for ridges on the inner 

 wall of the mantle (Rawitz, 1888; Kellogg, 1892). 



The structure of the accessory heart of C. 

 idrginica (fig. 238) resembles that of the arteries 

 of the mantle. The walls have a well-developed 

 layer of longitudinal and circular muscles, but the 

 endothelium lining is indistinct and is probably 

 absent. 



The pulsation of accessory hearts of C. virginica 

 observed in winter at the Woods Hole laboratory 

 was very irregular, not exceeding two to three 

 times per minute at room temperature of 20° to 

 22° C, and was independent of tlie heart beat. 

 During the summer the rate of contraction was six 

 to seven times per minute. Hopkins (1934) 

 states that in C. gigas the accessory heart beat 

 at a slower rate than the average heart rythm of 

 this species and the frequencies for right and left 

 organs averaged 6.0 and 7.5 times per minute 

 respectively. 



The connection of the accessory heart to other 

 vessels was studied by the following method of 

 injection. Live oysters were kept for 24 to 48 

 hours in a refrigerator, tlien placed overnight in 

 cold sea water with 5 percent magnesium sxilphate. 

 About 2 ml. of litliium carmine was injected, 

 using the finest iiypodermic needle. The prepara- 

 tion was rapidly rinsed in fresh water and im- 

 mediately immersed in 95 percent ethanol, which 

 precipitated the dye. The injected material 

 remained inside the vessels and was not diffused 

 or washed away by dehydration and clarifying 

 agents (cedar oil or xylene). In this way several 

 permanent preparations were obtained. 



Dye injected into the ventral branch (fig. 237, 

 v.br.) penetrated some distance into the circum- 

 pallial arteries of the right and left mantle lobes 

 and into the small branches and capillaries of the 

 efferent vein of tiie gills (ef.v.). The dorsal 

 branch (d.br.) was found to extend along the wall 

 of the cloaca: it does not "disappear into the 

 excretory organs," as stated by Hopkins, but 

 extends under the renal sinus to the dorsal part 

 of the cloacal wall. The ramifications of the 

 branch end in a number of capillaries which 

 connect them with the dorsal portion of the 

 efferent vein. The third or posterior branch 

 (p.br.) follows the ventro-lateral border of the 

 adductor muscle and gives many ramifications 

 inside the cloacal wall. 



Blood carried by the ventral branch of the ac- 

 cessory heart enters the pallial artery against the 

 pressure produced by the principal heart. Under 

 these conditions its penetration inside the artery 

 must be limited, and at the end of the contraction 

 wave some of the blood probably re-enters the 

 branch. Movement of the blood inside the cir- 

 cumpallial artery can not be seen, but through the 

 thin wall of the accessory heart one can observe the 

 flushing of blood cells back and forth. Ramifica- 

 tions of the ventral and dorsal branches form 

 capillaries which are in direct connection with the 

 side vessels of the efferent vein. It can be assumed 

 from the direction of the contraction waves that 

 blood from the accessory heart moves toward the 

 efferent vein of the gill and that part of the blood 

 is flushed back as the impulse wave progresses 

 along the wall of the branch. 



OsciUation of the blood in the mantle is the 

 primary function of the accessory hearts. Their 

 oscillatory movements facilitate the gaseous ex- 

 change and provide a means for efficient respira- 

 tion. The location of the accessory hearts con- 



CIRCULATORY SYSTEM AND BLOOD 



259 



