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 



