376 



THE CRAYFISH 



The heart (Fig. 93, 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 (o), guarded 

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

 pericardial sinus (Fig. 94, /<r)~which contains blood. From 

 the heart spring a number of delicate arteries (compare 

 P- 337)j 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 

 (Fig. 93, oa\ which 

 passes forwards to the 

 eyes ; paired anten- 

 nary arteries (a a), 

 going to the anten- 

 nules, 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 at their origin the 

 dorsal abdominal artery (o a a\ which passes backwards 

 above the intestine, sending branches to it and to the dorsal 

 muscles ; land the large sternal artery (Figs. 93 and 94, so), 

 which extends directly downwards, indifferently to right or 

 left of the intestine, passing between the connectives uniting 

 the third and fourth thoracic nerve-ganglia, and then turns 

 forwards and runs in the sternal canal, immediately beneath 



FIG. 95. Diagram of excretory oigan of Crayfish. 

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

 d. duct ; .?. yellowish sac-like portion ; ?c. /. 

 white tubular portion. (From Parker and 

 Haswell's Zoology, after Marchal.) 



