362 



THE CRA\TISH 



The heart (Fig. 86, h) is situated in the dorsal region of 

 the thorax, and is a roughly polygonal, muscular organ 

 pierced by three pairs of apertures or ostia (p), guarded 

 by valves which open inwards. It is enclosed in a spacious 

 pericardial sinus (Fig. 87, pc) which contains blood. From 

 the heart - spring a nurnber of narrow arteries (compare 

 p. 27), 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 only, 

 viz., from the heart 

 to the artery. From 

 the anterior end of 

 the heart arise five 

 vessels — the median 

 opthalmic artery (Fig. 

 86, oa), which passes 

 forwards to the eyes ; 

 paired anfennary ar- 

 teries {aa), going to 

 the antennules, an- 

 tennae, 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 at their origin— the dorsal abdominal 

 artery (p a a), which passes backwards above the intestine, 

 sending branches to it and to the dorsal muscles ; and the 

 large sternal artery (Figs. 86 and 87, sci), which extends di- 

 rectly downwards, indifferently to right or left of the intestine, 

 passing between the connectives uniting the third and fourth 

 thoracic ganglia, and then turns forwards and runs in the 

 sternal canal, immediately beneath the nerve-cord, sending 



Fig. 88. — Diagram of excretory oigan of Crayfish. 

 bl. bladder ; c. p, outer or cortical green portion ; 

 d. duct ; J-. yellowish sac-like portion ; w. p. 

 white tubular portion. (From Parker and 

 Haswell, after Marchal.) 



