278 MANUAL OF ELEMENTARY ZOOLOGY 



sinuses. The heart is divided into thirteen chambers 

 corresponding to the segments, and each chamber com- 

 municates by a pair of ostia at its sides with a pericardial 



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space, into which blood is returned from the periviscera 

 cavity through openings in its floor. Paired, triangular 

 alary muscles, whose outer ends are attached to the terga, 

 move the pericardial floor, and thus cause the flow of blood 



