130 CRAYFISH, LOBSTERS, SPIDERS, AND INSECTS 



either directly or indirectly with one large sinus, or canal, 

 that runs along the middle of the thorax and abdomen be- 

 low the intestine. From the anterior end of this large 

 sinus tubes conduct the blood to the different gills. Here, 

 the blood is carried in one tube to the gUl filaments, where 

 it is aerated and returned through the other gill blood 

 vessel to a set of veins, the branchio-cardiac veins, that lead 

 to the pericardium, a sac surrounding the heart. Finally 

 then, the blood which was sent out from the heart is re- 

 turned and poured into a cavity, in the middle of which 

 lies the heart. The blood now flows directly into the heart 



through six simple open- 

 y^^^^^antennc ings in the waUs of this 



Nerve ringj 

 aroundguUeT 



cBrairv 

 AGullet 



organ. The holes are all 

 provided with valves on 

 the inside that prevent the 

 return of the blood when 

 the heart contracts. There- 

 fore, the beating of the 

 heart sends the colorless 

 blood all over the body 

 and brings it back again 

 in a continual flow. 



The nervous system. — 

 The nervous system is 

 quite similar to that of the 

 earthworm (Fig. 71). It 

 consists of a brain formed from the fusion of three pairs 

 of ganglions and situated in the head anterior to the 

 stomach. A double white nerve cord extends from the 

 brain along the floor of the thorax and abdomen. As in 

 the earthworm, so in the crayfish, the cord passes on the 



Cq>halo- 

 thorax 



-Gatiglum 



^Abdomen 



dttne/lion, 



Fig. 71. — Nervous system of a crayfish. 



