\\!> PHYSIOLOGY 



109 



1'arN <>!' the l';i! hody ni;iy aUo he nix erne* I in excreti- tin- 



fat hody in Collemhola and ( )rt hoptci lor tin- |. 



o! u rat cs. 



7. CIRCULATOR KM 



In>ccts. unlike vertebrates. have n< system of closed hlood -vessels, 

 hut tin- hlood wanders freely through the hody 

 cavity to enter eventually the dorsal vessel, which 

 resembles a heart merely in being a propulsatory 

 organ. 



Dorsal Vessel. The dorsal vessel (Figs. 160, 

 104) is a delicate tube extending along the median 

 dorsal line immediately under the integument. 

 A simple tube in some larvae, it consists in most 

 adults chiefly of a series of chambers, each of which 

 has on each side a valvular opening, or ostium 

 (Fig. 161), which permits the ingress of blood but 

 opposes its egress; within the chambers occur 

 other valvular folds that allow the blood to move 

 f i >r ward only. With few exceptions (Ephemeridae) 

 the dorsal vessel is blind behind and the blood can 

 enter it only through the lateral ostia. 



Aorta. The posterior, or pulsating portion 

 ' heart) of the dorsal vessel is confined for the most 

 part to the abdomen; the anterior portion, or aorta, 

 extends as a simple attenuated tube through the 

 thorax and into the head, where it passes under 

 the brain and usually divides into two branches se i beetle, 

 (Fig. 164), each of which may again branch. In ao ? a al > alaf y muscle; . 



, * i ostium. After STRAUS- 



the head the blood leaves the aorta abruptly and DURCKHEIM. 

 enters the general body cavity. 



Alary Muscles. Extending outward from the " heart," or pro- 

 pulsatory portion, and making with the dorsal wall of the body a pericar- 

 dial chamber, is a loose diaphragm, formed largely by paired fan-like 

 muscles the alary muscles (Figs. 160, 162) . These are thought to assist 

 the heart in its propulsatory action. 



Structure of the Heart. The wall of the heart is remarkably thin, 

 and consists essentially of a muscular layer containing closely-set circular 

 or spiral libers and separated longitudinal fibers, with scattered nucle- 

 ated cells among the fibers. This muscular tube is between two layers: 





