378 GENERAL STRUCTURE AND FUNCTIONS 



and special sense ; the chest, upon which are concentrated the or- 

 gans of locomotion, the legs and wings ; and the abdomen, contain- 

 ing the greater part of the alimentary canal, together with the 

 glandular and generative organs. As the insects have a greater 

 amount of intelligence and activity than the centipedes and other 

 worm -like articulata, and as the organs of special sense are more 

 perfect in them, the cerebral ganglia are also unusually developed, 

 and are evidently composed of several pairs, connected by commis- 

 sures so as to form a compound mass. As the organs of locomo- 

 tion, furthermore, instead of being distributed, as in the centipede, 

 throughout the entire length of the animal, are concentrated upon 

 the chest, the locomotory ganglia also preponderate in size in this 

 region of the body ; while the ganglia which preside over the secre- 

 tory and generative functions are situated together, in the cavity of 

 the abdomen. 



All the above parts, however, are connected, in the same manner 

 as previously described, with the anterior or cerebral pair of gan- 

 glia. In all articulate animals, moreover, the general arrangement 

 of the body is symmetrical. The right side is, for the most part, 

 precisely like the left, as well in the internal organs as in the ex- 

 ternal covering and the locomotory appendages. The only marked 

 variation between different parts of the body is in an antero-pos- 

 terior direction ; owing to different organs being concentrated, in 

 some cases, in the head, chest, and abdomen. 



Finally, in the vertebrate type of animals, comprising man, the 

 quadrupeds, birds, reptiles, and fish, the external parts of the body, 

 together with the locomotory apparatus and the organs of special 

 sense, are symmetrical, as in the articulata ; but the internal organs, 

 especially those concerned in the digestive and secretory functions, 

 are unsymmetrical and irregular, as in the molluscs. The organs 

 of respiration, however, are nearly symmetrical in the vertebrata, 

 for the reason that the respiratory movements, upon which the 

 function of these organs is immediately dependent, are performed 

 by muscles belonging to the general locomotory apparatus. The 

 nervous system of the vertebrata partakes, accordingly, of the struc- 

 tural arrangement of the organs under its control. That portion 

 which presides over the locomotory, respiratory, sensitive, and in- 

 tellectual functions forms a system by itself, called the cerebro-spinal 

 system. This system is arranged in a manner very similar to that 

 of the articulata. It is composed of two equal and symmetrical 

 halves, running along the median line of the body, the different 



