18 INVERTEBRATE ZOOLOGY 
The circulatory system. This system is very simple in insects, 
owing to the great complexity of the respiratory system. In- 
stead of the blood being carried to the respiratory organs to 
be aérated, as is the case in vertebrates, rendering necessary 
a complicated system of blood-tubes connecting the remotest 
parts of the body with the respiratory organs, the respiratory 
organs are themselves a system of tubes which introduce air to 
every part of the body. The insect has a blood fluid which 
lies in the body-cavity. The only circulatory vessel present is 
the tubular heart. ‘This organ, whose position has already been 
noted, has a closed hinder end and segmental valvular openings 
along its sides. By its contractions the blood is sent into the 
forward portions of the body, whence it flows back into the 
hinder portions, and enters the heart again through the valvu- 
lar openings. To observe the heart of an insect is not always 
easy, owing to its position so near the dorsal body-wall 
and its great delicacy of structure. An. easy method .is to 
mount a live, transparent, aquatic insect larva, such as that 
of the mosquito, on a slide in water and observe it under a 
compound microscope. The heart and its action may be easily 
studied. 
The nervous system. Cut off the alimentary tract at its forward 
end, taking care not to injure the two nerve connectives which 
pass to the brain, and remove all the viscera from the body. 
The nerve cord will be seen lying on the ventral body-wall of the 
abdomen, in the median line, slightly concealed by fat. It will 
be seen to be double and to contain, in the abdomen, five 
enlargements, the ganglia, from each of which fine nerves radiate. 
Trace the nerve cord from the abdomen into the thorax. It is 
here protected by hard projections of the body-wall, which 
must be carefully removed. Four large ganglia will be found 
here, the three posterior ones of which are the thoracic ganglia. 
The one in the forward portion of the prothorax really belongs 
to the head and is called the subesophageal ganglion. From it 
