INSECTS 349 



never has the opportunity of remaining impure, a fact which ac- 

 counts to some extent for the imperfect condition of the circulatory 

 organs. The restless activity of insects, and their great muscular 

 powers, are also no doubt related to the unusually perfect state of 

 the breathing apparatus. 



Excretory Organs. — These also are very unlike anything so 

 far described in the animal groups reviewed. They consist of 

 a large number of very slender Malpighian tubes, which open 

 into the intestine not far from its commencement. 



Nervous System and Sense Organs (fig. 202). — The nervous 

 system consists of a narrow and much-thickened nerve-ring closely 

 encircling the gullet, and of a ventral double nerve-cord. The 

 upper part of the ring is formed by a pair of large brain-ganglia, 

 sending nerves to the eyes and antennae, while its lower part is 

 made up of another pair of ganglia, supplying the upper lip and 

 three pairs of jaws. The ventral cord dilates within the thorax 

 into three pairs of ganglia which innervate the three thoracic 

 segments, and within the abdomen into six smaller pairs of 

 ganglia, of which the first five belong to the corresponding 

 abdominal segments, while the last and largest pair provide for 

 the nerve-supply of the last five segments. Segmentation of the 

 body as exhibited by Vertebrates, Arthropods, and some other 

 phyla of the animal kingdom, means the existence of a number 

 of successive rings or segments, containing sections of the various 

 internal organs. Where the segmentation is very well marked, 

 many structures are affected by it, and the more primitive the 

 animal the more closely do the segments resemble one another. 

 As regards the nervous system of Arthropods, there can be no 

 doubt that the simplest condition is found in the presence of a 

 distinct pair of ganglia for each segment, but Insects especially 

 present considerable modifications of this primitive arrangement. 

 Where a segment is relatively large, its ganglia will be so too, 

 as seen in the thorax of the Cockroach, while fusion of segments 

 will be accompanied by fusion of ganglia, as in the case of the 

 jaw-bearing segments. The ganglia, further, may be among the 

 first parts to fuse together, as seen in the last pair of the abdominal 

 chain in the Cockroach, which represent the ganglia for the last 

 five segments all united together, though the segments themselves 

 are still more or less distinct. 



The chief sense organs are those of touch — including the 



