SEA-FISHERIES LABORATORY. 411 



pleural muscle chamber of the sixth thoracic segment. 

 All the pleural sinuses are below the branchio-cardiac 

 veins, which also run down the pleural muscle chambers. 



The Infra-branchial sinus (fig. 55, i. s.) is a long 

 sinus which runs along each side of the thorax at the 

 base of the thoracic legs and the gills. Posteriorly it 

 extends as far as the last walking leg, and anteriorly 

 almost as far as the metastoma. Into it flow the five 

 branchial sinuses from above, and from its outer and ventral 

 side there enters a narrow sinus from each of the thoracic 

 appendages. From its outer and dorsal side there is 

 given off an afferent branchial sinus to each of the gills. 



Afferent Branchial sinuses (fig. 55). These sinuses 

 run along the outer side of each gill. 



First afferent branchial sinus (af . 1) goes to the podo- 

 branch of the second thoracic somite. 



Second afferent branchial sinus (af . 2) goes to the 

 arthrobranch of the second thoracic somite. 



Third afferent branchial sinus (af . 3) goes to the small 

 podobranch of the third thoracic somite. 



Fourth afferent branchial sinus (af . 4) goes to the 

 anterior arthrobranch of the third thoracic somite. 



Fifth afferent branchial sinus (af. 5) goes to the 

 posterior arthrobranch of the third thoracic somite. 



Sixth afferent branchial sinus (af. 6) goes to the 

 anterior arthrobranch of the fourth thoracic somite. 



Seventh afferent branchial sinus (af . 7) goes to the 

 posterior arthrobranch of the fourth thoracic somite. 



Eighth afferent branchial sinus (af . 8) goes to the 

 pleurobranch of the fifth thoracic somite. 



Ninth afferent branchial sinus (af. 9) goes to the 

 pleurobranch of the sixth thoracic somite. 



The Efferent Branchial veins (fig. 54) receive the 

 pure blood from the branchial lamellae. They pass down 



