208 



THE CRAYFISH. PHYLUM ARTHROPODA 



around the stomach, however, passes on each side into the 

 space between the two sides of the fold of carapace which forms 

 the branchiostegite, and thence to the pericardial sinus by a vessel 

 which follows the hinder edge of the branchiostegite. It will be 

 noted that the pericardial cavity of the crayfish is a part of the 

 hitmocctle and contains blood, unhke that of the vertebrates, 

 which is a separate part of the ccelom. A blood-vascular system 



.hi. 



.OS. ,'pcrn. 



ext.m. 



,eff.hY,s. 



ov. -_ 



' jLm. 



v.n.c.'^'^^-^- '*'•'• 



'w.l. 



Fig. 140. — A diagram of a transverse section through the thorax of a crayfish. 



urb., Arthrobranch ; hr ., outer layer of branchiostegite ; br"., inner layer of the same ; eff.br.s., efferent 

 branchial sinus ; en.sk., endophragmal skeleton ; ext.m., extensor muscle of abdomen ; fl.nt., flexor 

 muscles of abdomen ; g.c, gill-chamber ; h.g., hind-gut ; ht., heart ; Ir., liver ; os., ostia ; ov., ovary ; 

 Pcm., pericardium ; pbr., podobranch ; plb., 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. 



in which, as in the crayfish, the blood on leaving the arteries 

 bathes the organs of the body is said to be open. One in which, 

 as in the worm and the vertebrates, it is carried through the 

 organs in capillaries which lead direct to veins is said to be 

 closed. The blood of the crayfish is a clear fluid, which contains 

 white corpuscles and clots readily — an obvious advantage to an 

 animal whose open vascular system causes it to bleed freely 

 from any wound. A respiratory pigment known as haemocyanin, 

 which is an organic compound of copper, is dissolved in it, and 

 plays the same part as haemoglobin, taking up oxygen in the 



