XI 



PHYLUM ARTHROPODA 



641 



mesenteron are five pairs of narrow tubes (Figs. 546 and 547, 

 hep. du.) leading into the substance of a large glandular body, 

 usually termed the liver (hep.), though its hepatic functions are 

 doubtful. Into the long narrow intestine, in the last segment of 



the pre-abdomen, open one or two pairs of delicate tubes the 



Malpighian tubes (mal.) which act as the organs of renal excretion. 



Circulatory organs. An elongated tubular heart (Fig. 546, 



hrt.) lies in the pre-abdomen enclosed in a pericardial sinus ; it is 



