72 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 



traverses the nervous cord (figs. 12 and 15), and divides 

 into an anterior (fig. 12, sa) and a posterior {iaa) branch, 

 both of which run beneath and parallel with that cord. 



FlG 16. — Astacus fluviatilis. — The heart ( x 4). A, from above ; B, from 

 below ; C, from the left side, aa, antennary artery ; ac, alse cordis, 

 or fibrous bands connecting the heart with the walls of the peri- 

 cardial sinus ; b, bulbous dilatation at the origin of the sternal 

 artery ; lm, hepatic artery ; la, lateral valvular apertures ; na, oph- 

 thalmic artery ; s.a, superior valvular apertures ; s.a.a, superior 

 abdominal artery ; st.a, sternal artery, in B cut off close to its 

 origin. 



A third artery runs, from the front part of the heart, 

 forwards in the middle line, over the stomach, to the 

 e)'es and fore part of the head (figs. 5, 12, and 16, oa)\ 

 and two others diverge one on each side of this, and sweep 



