552 



ZOOLOGY 



thorax, and is a roughly jJolygonal muscular organ pierced by 

 throe pairs of a23erturos or ostia (o.), guarded by valves which open 

 inwards. It is enclosed in a spacious pericardial sinus (Fig. 438, 

 pc), which contains blood. From the heart spring a number of 

 narrow tubes, called arteries, which serve to convey the blood to 

 various parts of the body. At the origin of each artery from the 

 heart are valves which allow of the flow of blood in one direction 



Fig. 43V.— Diagi-am of kidney of Astacus fluviatiUs. I, unravelled ; II, the parts in their 

 natural relations, hi. bladder; f:. p, cortical portion; d. duet; s. saccule; w. /i. white 

 portion. (After Marchal.) 



only, viz., from the heart to the artery. From the anterior end 

 of the heart arise five vessels — the median ophthalmic artery 

 (Fig. 435, oa.), which passes forwards to the eyes ; paired ' an- 

 tcnnary artc7-ies (a.a.), going to the antennules, antennae, green 

 glands, &c., and sending off branches to the gizzard ; and paired 

 hepatic arteries, going to the digestive glands. The posterior end 

 of the heart gives off two unpaired arteries practically united 



