28 CIRCULATORY ORGANS. 



necessary for the preparation of the food. Of this 

 nature are the salivary glands which discharge their 

 secretion either into the mouth or into the oesophagus 

 close to the stomach, and the biliary vessels which 

 take the place of the liver in these animals, and open 

 into the intestine behind the stomach. All these 

 glands are usually of a tubular form and very simple 

 in their construction ; they either terminate in a closed 

 extremity, or are united in pairs so as to form a long 

 loop. Other secreting organs are found near the 

 anus ; their secretion is analogous to the urine of the 

 higher animals, and their structure varies consider- 

 ably, but it will be unnecessary here to do more than 

 mention their existence. 



With all this complexity in the structure of the 

 organs by which the nutritive matter is prepared for 

 admission into the fluids of the body, we find the 

 apparatus by which the circulation of these fluids is 

 maintained to be of a most simple description. The 

 representative of the heart in Insects, is a delicate 

 vessel which runs all along the back from the hinder 

 part of the abdomen to the head. This vessel, like 

 the body of the insect itself, is divided into segments 

 by the folding in of the skin of which it is composed, 

 and close to each of these folds there is an aperture in 

 the side of the vessel, through which it communicates 

 with the general cavity of the body. This is the 

 structure of the heart (or dorsal vessel as it is called), 

 whilst in the abdomen, but it passes through the 

 thorax in the form of a simple tube, which terminates 

 with a small orifice at the head. It is suspended in 

 its place by broad triangular muscles attached to the 

 back of the abdomen, but these seem to have little or 

 nothing to do with its contractions, which are effected 



