CRUSTACEANS 



59 



The Circulation. — The blood is a liquid containing white cor- 

 puscles. It lacks red corpuscles and is colourless. The heart is in 



the upper part of the thorax. It is sur- 

 rounded by a large, thin bag, and thus it is 



in a chamber (called the pericardial sinus). 



The blood from the pulmonary veins enters 



this sinus before it enters the heart. The 



origin of this pericardial sinus by the fusing of 



veins is shown in Fig. 1 30. Does one artery, 



or do several arteries, leave the heart ? There 



is a larger dorsal artery lying on the intestine 



and passing back to the telson ; there are 



three arteries passing forward close to the 



dorsal surface (Figs. 89, 91). One large artery 



(the sternal) passes directly downward (Figs. 



88, 91), and sends a branch forward and 



another backward near the ventral surface. 



The openings into the heart from the sinus 



have valvular lips which prevent a backward 



flow of blood into the sinus. Hence, when 



the heart contracts, the blood is sent out into the sev- 

 eral arteries. The arteries take a supply of fresh blood 

 to the eyes, stomach, muscles, liver, and the various 

 organs. After it has given oxygen to the several organs 

 and taken up carbon dioxide, it returns by veins to pass 

 through the gills on each side, where it gives out the use- 

 less gas and takes up oxygen from the water. It is then 

 led upward by veins into the pericardial sinus again. 

 The central nervous system consists of a double 

 chain of ganglia (Fig. 92). This main nerve chain 

 lies along the ventral surface below the food tube 

 (Fig. 90), except one pair of ganglia which lie 

 above the oesophagus or gullet (Fig. 88), and are 

 called the supra-cesophageal ganglia, or brain. 



Fig. 91. — Showing heart 

 and main blood vessels. 



Crustacea — The crayfish and its kindred are placed in 

 the class called Crustacea. 



