CIRCULA TOR Y S YSTEM. 34) 



Circulatory system. As the respiratory system is very 

 efficient, air passing into the inmost recesses of the body, 

 it is natural that the blood-vascular system should not be 

 highly developed. Within a dorsal part of the body cavity, 

 known as the pericardium, the heart lies, swayed by special 

 muscles. It is a long tube, usually confined to the abdomen, 

 and with eight chambers, with paired valvular openings on 

 its sides, through which blood enters from the pericardium. 

 The blood is driven forwards, the posterior end of the heart 

 being closed, and there is usually an anterior aorta or main 

 blood vessel. But, for the most part, the blood circulates 

 in spaces within what is commonly called the body cavity. 

 Such a circulation is often described as lacunar. The blood 

 may be colourless, yellow, red, or even greenish, and, in 

 some cases, haemoglobin, the characteristic blood pigment of 

 Vertebrates, has been detected. The cells of the blood are 

 amoeboid. 



Body cavity. It is necessary to distinguish the primitive coelom 

 from the apparent body cavity of the adult. In discussing the develop- 

 ment of PeripatuS) Sedgwick notes the following characteristics of a 

 true coelom : It is a cavity which (i) does not communicate with the 

 vascular system; (2) does communicate by nephridial pores with the 

 exterior; (3) has the reproductive elements developed on its lining; 

 (4) develops either as one or more diverticula from the primitive 

 enleron (or gut), or as a space or spaces in the unsegmented or 

 segmented mesoderm. Now, in Arthropods the apparent body cavity 

 of the adult is not a true coelom : it consists of a set of secondarily 

 derived vascular spaces ; it has been called a pseudoccel or a haemoccel. 

 The true coelom of Arthropods is very much restricted in the adult. 



The apparent body cavity in which the organs lie, and in which 

 the blood circulates, is well developed in Insects. It includes, inter 

 alia, a peculiar fatty tissue, which seems to be a store of reserve 

 material, which is especially large in young insects before metamorphosis, 

 and is also interesting as one of the seats of " phosphorescence." 



Excretory system. Although no structures certainly 

 homologous with nephridia have yet been demonstrated in 

 Insects, the excretory system is well developed. From the 

 hind-gut (proctodasum), and therefore of ectodermic origin, 

 arise fine tubes, or in some cases solid threads, which extend 

 into the apparent body cavity. Their number varies from 

 two (in some Lepidoptera, for instance) to one hundred and 

 fifty (in the bee). They twine about the organs in the 

 abdominal cavity, and their excretory significance is inferred 

 from the fact that they contain uric acid. 



