CIRCULATORY SYSTEM. 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, hemoglobin, 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 ccelom 
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 isa cavity which—(1) 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 
enteron (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 ccelom: it consists of a set of secondarily 
derived vascular spaces ; it has been called a pseudoccel or a heemoccel. 
The true ccelom 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, zter 
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 (proctodzeum), 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. 
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