THE CRAYFISH 



255 



the intestine and supplies it and the muscles of the 

 abdomen, and a sternal artery. This passes downwards, 

 through an opening in the ventral nerve cord, and divides 

 into a ventral abdominal and a ventral thoracic artery, 

 by which the limbs are supplied. Each of the arteries 



,fct. 



AS- ,pcm. 



arb. 



-fl.m. 



v.tha- <"•'• 



Fig. 162. — A diagram of a transverse section through the thorax of 

 a crayfish. 



arb,, Arthrobranch ; br 1 ., outer layer of branchiostegite ; £r"., inner layer of the 

 same ; cff.br. s., efferent branchial sinus ; en.sk., endophragmal skeleton ; ext.m., 

 extensor muscle of abdomen; fl.m., flexor muscles of abdomen; g.c, gill- 

 chamber; k.g. t hind-gut ; kt., heart; lr., liver; os., ostia; ov., ovary; J>cm., 

 pericardium ; pbr., podobranch ; $lb., pleurobranch ; st.s., sternal sinus ; v.n.c, 

 ventral nerve cord ; v.th.a., ventral thoracic artery ; ■w.l., walking leg. 



Small arrows in the sinuses on the right-hand side show the course of the circulation 

 of the blood. 



branches many times, till it finally gives rise to minute 

 vessels in the organs it supplies, but there are no capillaries. 

 It will be seen that in the crayfish, as in the earthworm, 

 there is a dorsal contractile blood vessel, but that in the 

 crayfish the contractile organ — the heart — is very short and 



