328 ZOOLOGY 
muscles, which correspond in number with the cardiac cham- 
bers, and have a wedge-shaped outline, are attached at one 
end to the integument, and at their broader extremity to the 
pericardial membrane; when they contract the latter is de- 
pressed. This membrane consists of connective tissue, pierced 
by numerous oval apertures; when it is depressed the blood in 
the body-cavity passes through it, and at the same time the 
diastole of the heart taking place, the blood enters through the 
eight pairs of ostia, and at the systole is forced forward and so 
out of the open mouth into the body-cavity again. In this 
way the blood, which is a colourless fluid with amoeboid 
corpuscles, is kept in circulation. 
The body-cavity of Insects is to a great extent occluded by 
the various viscera, but in addition to the alimentary canal, 
generative organs, etc., there is a considerable amount of a 
tissue, known as the fat-body, which is formed primitively 
from mesoblast cells lining the integument. This fat-body is 
especially abundant in the larvae, where to some extent it acts 
as a storehouse for reserve material, particularly in those 
Insects which pass through a protracted pupa stage; it is also 
found in mature Insects, and is usually present to a greater or 
less extent on the pericardial membrane. In the Tracheata, 
where the air is directly conveyed to the cells of all the tissues, 
the blood has to a great extent lost its respiratory function ; it 
is still, however, of the utmost importance. It bathes all the 
internal organs, and these, as is usually the case when organs 
are surrounded by nutrient media, do not form solid compact 
masses, but are branched and subdivided as much as _ possible. 
The food which has been digested in the alimentary canal is 
thus distributed by the blood into which it passes, the fats are 
stored up by the fat-body, and the nitrogenous excreta, the 
urates or uric acids, are either conveyed straight to the Mal- 
pighian tubules, or are stored up in the cells of the fat-body. 
From time to time these cells break down, and then the stored- 
up urates are taken by the blood to the Malpighian tubules, 
and from them pass out of the body. The body-cavity in 
Insects, as is probably the case in all Arthropods, is a haemo- 
coel, and the true coelom is probably confined to the lumen of 
the generative organs. The developement of a tracheal system 
