VASCULAR RESPIRATORY ORGANS. 



533 



At 



unfrequently has a green tinge, always contains amoeboid blood cells 

 and travels along definite tracts of the body cavity. The simplification 

 of the circulator// </]tj><trnfiift, which is confined to a </o/W vessel, is 

 correlated with the richly branched respiratory apparatus, the air- 

 conducting tmcheai, which are distributed to all the organs and carry 

 oxygen to the blood. The heart, which has the form of a dorsal 

 vessel (fig. 442), runs in the middle line of the abdomen, and is 

 divided by transverse constrictions into numerous (up to eight) 

 chambers corresponding to the seg- 

 ments. These chambers are attached 

 to the integument of the dorsal sur- 

 face by triangular muscles (alary 

 muscles). During the diastole of 

 the chambers the blood streams 

 through as many paired lateral 

 slits into the heart, which contracts 

 gi-.-i dually from before backwards 

 and drives the blood in the same 

 direction. The anterior chamber 

 is prolonged into a median aorta, 

 which runs forward to the head. 

 From this aorta the blood flows 

 freely into the body cavity and 

 returns to the heart in four prin- 

 cipal streams, two lateral, one dorsal 

 beneath the dorsal vessel, and one 

 ventral above the ganglionic chain, 

 giving off numerous branches to the 

 extremities, etc. It is only in ex- 

 ceptional cases (e.(j., in the caudal 

 filaments of the larva? of Ephemera) *"> m-Longitudmai section through 



the body of SpMnx UquM (after 



that arterial vessels are found pass- Newport). 

 ing out from the heart. 



Respiration is effected by branched 

 trachew, which take in their supply 

 of air through paired slit-like open- 

 ings, the stiijiiintit. The latter are 

 usually situated in the membranes 

 terga (fig. 428), and the exchange of air is determined by the 

 distinct respiratory movements of the abdomen. The number of 

 stigmata is very various, but there are rarely more than nine or fewer 



MX, maxilla 1 forming the 

 proboscis ; /, palp ; At, antenna ; (;.-, 

 brain ; Gi, subcesophageal ganglion ; 

 JV, thoracic and abdominal g;inglia; 

 7", oesophagus ; 7"', suctorial stomach ; 

 M, mesenttsron ; V,, Malpighian tubes; 

 //, heart ; G, testes ; E, rectum ; .4, 

 amis. 



connecting 



the sterna and 



