lol) CHAYFISII, LOB.STKIJS, si'IDKliS, ANJ) INSECTS 



Neroe ring, 

 aruilndyuUi 



either directly or inclirecfly with une large sinus, or canal, 

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

 low the intestine. From the anterior end of this large 

 sinus tul)es conduct the blood t(j the diffeicnt gills. Here^ 

 the blood is cai'ried in one tulje to the gill filaments, where 

 it is aerated and returned through the otlier gill blood 

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

 to the pericardium, a sac surroumhng 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 tlirectly into the heart 



through six simple open- 

 ings in the walls of this 

 organ. The holes are all 

 provided with valves on 

 the inside that pre\"ent 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 

 eai-thworm (Fig. 71) It 

 consists of a brain formed from the fusion of three pau's 

 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 



Optic nerve 



Nerve to antenna^ 

 Tirairh 

 Gullet 



C'ephalo- 

 thorax 



Ganglion, 



'Ahdon 



Ganglion 



Fig. 71. — Nervoiu .■^ysteiu of a crayfish. 



