376 



THE CRAYFISH 



CHAP. 



ment. The heart (Fig. 93, //) is situated in the dorsal 

 region of the thorax, and is a roughly polygonal, muscular 

 organ pierced by three pairs of apertures or ostia (o), 

 guarded by valves which open inwards. It is enclosed 

 in a spacious pericardial sinus (Fig. 94, PC] which contains 

 blood. From the heart spring a number of delicate 

 arteries (compare p. 337), 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 

 ophthalmic artery 



C.f> 



FIG. 95. Diagram of excretory organ of Crayfish. 

 ( X 5.) bl. bladder ; c. p. outer or cortical green 

 portion ; d. duct ; s. yellowish sac-like por- 

 tion ; w. p. white tubular portion. (From 

 Parker and Haswell's Zoology, after Marchal.) 



(Fig. 93, oa], which 

 passes forwards to 

 the eyes ; paired 

 antennary arteries 

 (aa) , going to the an- 

 tennules, antennae, 

 green glands, &c., and sending off branches to the 

 gizzard ; and paired hepatic arteries, going to the diges- 

 tive glands. The posterior end of the heart gives off 

 two unpaired arteries practically united at their origin 

 the dorsal abdominal artery (oaa), which passes back- 

 wards above the intestine, sending branches to it and 

 to the dorsal muscles ; and the large sternal artery 

 (Figs. 93 and 94, sa), which extends directly downwards, 

 indifferently to right or left of the intestine, passing 

 between the connectives uniting the third and fourth 

 thoracic nerve-ganglia (p. 379), and then turns forwards 

 and runs in the sternal canal, immediately below the 



