THE HEART AND HAEMATOCOELE 129 



directly inwards across the dorsal wall, and unite with one another 

 in the middle line below the diaphragm. 



The mechanism of the heart is as follows. When the muscular 

 fibres in its wall contract, the openings by svhich the chambers com- 

 municate with the body cavity are closed ; as the 



. . . . . Mechanism 



blood is compressed it passes forwards, retrogression 



being prevented by the valves which separate the chambers of the 

 heart from one another. The heart is thus emptied into the aorta, 

 and a forward flow of blood brought about. 



The heart muscle then relaxes, and at the same time the alar muscle 

 contracts ; its contraction increases the space between the diaphragm and 

 the dorsal wall, and causes the blood to enter the space through the 

 apertures in the diaphragm, and thus to circulate among the pericardial 

 cells in which the heart lies. Probably the muscles of the abdominal 

 wall also assist by compressing the abdominal contents to a slight 

 extent. Contraction of the heart then commences again, and in this 

 way an intermittent stream of blood is kept flowing from behind 

 forwards ; all the blood circulates among the pericardial cells as it 

 passes from the abdomen to the heart. The pericardial cells are 

 believed to have some special function in purifying the blood, but in 

 view of the little that is known of insect metabolism it is idle to specu- 

 late as to what this may be. 



The body cavity has already been mentioned in connection with the 

 heart, as the space from which the blood is drawn into the contrac- 

 tile chambers. It is worth a little more attention 



.. . . . , i The Haematocoele 



than is usually paid to it by parasitologists, on ac- 

 count of its relations to all the organs of the body, and especially to 

 the proboscis. 



When an insect is opened up by removing the dorsal or ventral wall, 

 the internal organs are seen lying free in a space bounded only by the 

 exo-skeleton and the layer of cells immediately internal to it ; they 

 have no attachments other than those formed by the tracheae. This 

 space is the haematocoele. In the living insect it is filled up by 

 the blood, which bathes all the internal structures and is kept 

 moving by the action of the heart. The stream of blood passes 

 forwards in the dorsal tube, and, leaving it at the termination 

 of the aorta in the head, it returns to the abdomen in two lateral 

 streams, which divide up into numerous streamlets as they pass among 

 the contents of the cavity. Now the lumen of the alimentary tract is 

 only separated from the blood in the haematocoele by the thickness 

 17 



