690 



PROCEEDINdS OF THE NATIONAL MUSEUM. 



At every pulsation also a portion of the blood is driven backward 

 through the posterior median opening into the sinus around the cloaca. 

 The bulk of this stream passes directly back to the anus where it 

 divides, a half passing around the wide sinus on the border of each 

 lobe of the abdomen. 



But portions of the central cloacal stream are given off sidewise all 

 the wav back to the anus; these percolate through the sinuses and 

 lacunae and around the sexual organs of either lobe and finally join 

 the returning streams along either mar- 

 gin and enter the heart through the pos- 

 terior valved lateral openings.. 



In this way a constant interchange of 

 the blood from various parts of the body 

 is maintained, and that portion of it 

 which percolates through the lateral 

 areas of the carapace and the lobes of 

 the abdomen is thoroughl}- purified dur- 

 ing its passage. 



The circulation in americanm differs 

 in several particulars; there are but five 

 openings in the heart, the ventral one 

 consistingof a single median longitudinal 

 slit instead of paired lateral ones (fig. 

 18). All the blood enters through the 

 lateral valved openings a; a part of it 

 passes out of the aorta anteriorly h and 

 another part out of the posterior median 

 aperture d, and each of these streams 

 follows the same course as in foliaceiis. 

 But the greater bulk turns downward 

 and passes out of the median ventral slit 

 {(■). This stream pours around the 

 intestine and separates naturally into 

 two side streams, running forward past 

 the bases of the swimming legs, sending 

 out lateral streams into each of them, and joining the anterior streams 

 from the aorta underneath the brain. On its return the blood perco- 

 lates through the lateral sinuses of the carapace and, joining the 

 streams from the borders of the abdomen, enters the openings at the 

 sides of the triangular base of the heart. (See also fig. 19.) 



This circulation does not depend for its impetus upon the pulsation 

 of the heart alone. It is helped, as such lacunal circulation must 

 always be, by the contraction of muscles in various parts of the body. 

 Especially is this true of those muscles which contract somewhat 

 rhythmicall}^ like the muscles of the stomach and nitestine in their 

 peristaltic movements, and of the legs in swimming. There is also a 



Fig. 18.— Dorsal view of the heart 



OF ARGULUS AMEKICANl'S (DIAGRAM- 

 MATIC), a., Paired lateral open- 

 ings; b., ANTERIOR opening OF AORTA; 

 C, anterior, ANDrf., POSTERIOR VEN- 

 TRAL OPENING. 



