THE CIKCULATOEY SYSTEM. 



Ill 



wide channels against the lateral walls of each segment. It does not 

 run out free into the abdominal cavity, however, unless through a 

 rent, nor does it enter the latter from the thorax except by way of 

 the ventral sinus. The dorsal circulation of course can not be 

 observed in this specimen because the back is removed. Therefore, 

 take another bee and fasten it in the same manner, but make simply 

 a shallow median slit along the back, thus exposing the dorsal sinus 

 and the heart from above. Now insert a drop of stain into the 

 thorax as before. After about two minutes this will appear in jper- 

 ceptible amount in the dorsal sinus, very much diluted, to be sure, 

 with the blood, but there will be enough to give white blotting paper 

 a distinct red tint. In a short time the heart becomes filled with 

 the stained blood and appears as a red tube along the median line. 



The dorsal sinus contains not only the heart but also two pairs of 

 pericardial air sacs in each segment. These are seen entering the 



Fig. 49. — Pericardial chamber of one segment in worljer, seen from below looking through 

 transparent dorsal diaphragm {DDph), showing median heart (Ht), lateral pericardial 

 air sacs (BtTraSc) given off from large lateral sacs (TraSc), and the padding of 

 pericardial cells (HtCla) against inner surface of tergum (T) . 



sinus from the large lateral air sacs of the abdomen (TraSc) in 

 figure 1 and also in figure 47. In the latter the heart (Ht) is seen 

 along the median line as it shows through the diaphragm. Figure 

 49 gives a view of the pericardial sinus as seen from below, in one 

 segment, by focusing through the transparent diaphragm (DDph). 

 In the middle lies a chamber of the heart (Ht) with the slitlike 

 ostium on each side. Laterally are the four pericardial air sacs 

 (HtTraSc) giving off branches that ramify profusely upon the 

 heart. Above the heart and the air sacs is a thick bed of large 

 granular cells {HtCls) which make a soft padding between the hard 

 tergal wall and the delicate organs of the sinus. These are called 

 the pericardial cells. They may have some physiological function, 

 as has often been supposed, but if so no one has decided what it is. 



