120 



These Sternal sinuses pass backward at each side of the 

 ventral part of the thorax between the leg muscles and 

 beneath the pericardium. The two sternal sinuses are 

 not definitely separated from one another, but are 

 connected here and there by irregular sinuses. On the 

 outer side each sternal sinus sends offshoots down the 

 pleural muscle chambers to the base of the gills (fig. 56, 

 br. S. 4). These branches or branchial sinuses again unite 

 into a long sinus which runs along the base of the gills. 

 This is the infra-branchial sinus (fig. 55, i. s.). The blood 

 sinuses from each of the thoracic legs also pass into the 

 infra-branchial sinus. 



From the infra-branchial sinus the blood passes along 

 the afferent branchial sinuses on the outside of each gill. 



At the posterior end of the thorax the sternal sinuses 

 are connected with a small abdominal blood sinus. 



The sinuses in connection with the digestive gland — 

 the Hepatic sinus and the reproductive organs — the 

 Ovarian [or Spermatic] sinus — open into the sternal sinus 

 at each side. There is a very large sinus below the 

 posterior part of the digestive gland. 



The Branchial sinuses (br. s. 1-5) connect the 

 sternal sinus with the infra-branchial sinus at each side. 

 They are five in number at each side. The first branchial 

 sinus (br. s. 1) commences below the anterior end of the 

 pericardium. It passes down the pleural muscle 

 chamber of the second thoracic segment and opens 

 into the infra-branchial sinus near its anterior end. 

 Similarly the second branchial sinus (br. s. 2) passes 

 down the third thoracic pleural muscle chamber, the 

 third sinus (br. s. 'A) is in the fourth pleural chamber, the 

 fourth sintis (br. s. 4) is in the fifth pleural chamber, and 

 the last branchial sinus (br. s. 5), which commences below 

 the posterior end of the pericardium, passes down the 



