Ill 



the various gill lamellae where it is oxygenated. The 

 pure blood leaves the gills by the Efferent Branchial 

 Veins, running along the inside of the gills and which 

 pass into the Branchio-cardiac veins. 



The Pericardii! m (PI. IX, figs. 54, 56, Per.) is 

 a closed cavity surrounding the heart and having thin 

 transparent walls. It is situated immediately beneath 

 the cardiac region of the carapace, and between the 

 " flancs." It lies above the hind-gut and covers the 

 posterior portions of the digestive gland and gonads. 

 When viewed from above the shape of the pericardium 

 is roughly pentagonal (fig. 54). The base of the pentagon 

 is anterior and the apex is posterior. 



The Branchio-cardiac veins (PI. IX, fig. 54, he 1-5) 

 enter each side of the pericardium by means of three wide 

 openings which have no valves. The first opening is 

 situated at the anterior corner of the pericardium and 

 receives the first and second branchio-cardiac veins. The 

 second opening is situated a little behind the first and 

 receives the third branchio-cardiac vein. The third 

 opening is situated at the postero-lateral corner, and at 

 this point the fourth and fifth branchio-cardiac veins 

 enter the pericardium. 



The Heart (PI. VII, figs. 49, 50) is a white semi- 

 transparent body, pentagonal in shape when seen dorsally 

 and having a rectangular shape when viewed from the 

 side. Two angles of the pentagon are anterior and the 

 other three are posterior. The heart is suspended in the 

 pericardium by means of the alae cordis (fig. 49, 50, 

 cd.l-G), which are bands of fibrous connective tissue 

 stretching across from the angles of the heart to the wall 

 of the pericardium. Each ala cordis appears to have a 

 short band of muscle fibres attached to its outer 

 extremity. 



