240 ARTHROPODA [CH. 



and ten abdominal segments, each chamber opening into the one in 

 front and the whole somewhat resembling a row of funnels fitted one 

 into another. At the broader hinder end of each chamber is a pair 

 of ostia or holes through which the blood enters from the peri- 

 cardium, and there are valves which prevent the blood being forced 

 out of the ostia or forced backward when the heart contracts, so 

 that its only course is to flow forward. The pericardium is separated 

 from the rest of the haemocoele by the pericardial septum, in which 

 however there are certain holes which permit an interchange of con- 

 tents between the two cavities. When at rest the pericardial septum 

 is arched upwards, but it is palled outwards and flattened by the 

 periodic contraction of certain muscles attached to its sides called 

 the alary muscles, thus enlarging the pericardium and causing an 

 inflow of blood into it from the rest of the haemocoele. This blood 

 enters the heart when it is relaxed and the ostia are open, and hence 

 by the alternate contractions of the alary muscles and the muscular 

 wall of the heart the circulation is maintained. 



The anterior end of the heart, called the aorta, opens by a 

 trumpet-shaped orifice into the haemocoele and the blood pours out 

 of this and bathes all the organs of the body. It thus takes up the 

 soluble food which has left the alimentary canal and conveys it to 

 those parts of the body where it is needed, and in a similar way it 

 yields up its superfluous fat to be stored up in the fat-body and 

 gives up its waste nitrogenous materials to the Malpighian tubules, 

 whence they are passed out of the body. The haemocoele, especially 

 in the abdomen, is largely blocked by a peculiar development of 

 connective tissue, the cells of which are gorged with drops of fat 

 and which is consequently white and opaque. It is this tissue 

 which is termed the fat -body. 



The heart contracts almost as frequently as the normal human 

 heart, i.e., about seventy- two times a minute when the cockroach is 

 at rest, but at other times its rate of contracting varies a good deal. 

 The blood is colourless and contains amoeboid cells. It is slightly 

 alkaline. 



It is obvious from the above account that in the cockroach the 

 blood is mainly a means of conveying nutriment to the organs and 

 of taking certain waste matter from them, and that unlike what is 

 usual in other animals, its respiratory function is at a minimum. 

 Owing to the nature of the tracheae, the air with its oxygen is 

 taken directly to each organ, almost to each cell, without the inter- 

 vention of the blood. 



