TYPE CRUSTACEA. 



377 



trenrity encroaching but slightly upon the thorax (Isopoda). 

 It is provided with a varying number of openings along its 

 sides, through which the blood gains entrance to its cavity 

 from the pericardial sinus these openings, termed ostia, 

 being guarded by valves opening inwards and preventing 

 regurgitatiou of the blood during systole. From either end 

 of the heart arteries arise which, after a longer or shorter 

 course and many or few branchings, open widely into the 

 lacunar spaces. From these the blood passes in some forms 

 into a venous sinus situated on the ventral surface of the 



sa 



Vd vn 



t,K an 



FIG. 168. DIAGRAM OF STRUCTURE OP CRUSTACEAN (Cambarus). 



ne = nephridium. 



s = stomach. 

 sa = sternal artery. 

 te testis. 

 tl = telson. t 

 tid = vas deferens. 

 vn = ventral nerve. 

 1-6 = abdominal segments. 



an auus. 



ca carapace. 



ce = cerebral ganglion. 



h = heart. 

 i intestine. 

 I digestive gland. 

 TO mouth. 

 mp = opening of vas deferens. 



body, and thence is distributed to the branchiae, passing from 

 them back to the pericardial sinus, and so to the heart again. 

 The blood is usually colorless, though occasionally greenish, 

 in which case it contains a respiratory copper-containing pig- 

 ment termed hsemocyauin, or reddish, in which case the pig- 

 ment is haemoglobin. It consists of a plasma in which float 

 amoeboid nucleated corpuscles. 



The digestive system consists of an almost straight tube 

 extending from mouth (Fig. 168, m) to anus (an) and divisible 

 into three regions. The mouth is bounded in front by an 

 overhanging lip, and behind by a lower lip which arises as 

 two separate parts, which by some writers have been regarded 



