AND THE LOWER ANIMALS. 65 



small and is composed of two smooth lobes which are 

 obliquely united, and give off a few slender nervous 

 filaments. 



Two days after the caterpillar has been converted 

 into the chrysalis, the ganglionic chain has been 

 shortened one-fourth, and various changes of alteration 

 and concentration have begun. Some ganglia are 

 approximated, others, on the contrary, are separated. 

 At about the eighth day the chain has been shortened 

 one-half. On the fourteenth the brain and first gan- 

 glion have come so close together that their commis- 

 sures surround the oesophagus; the fourth and fifth 

 ganglia have been fused together ; and the sixth and 

 seventh are hardly perceptible. Now there is a period 

 of rest, brought about by the approach of winter. 

 Then, after the latter season, the operations are re- 

 commenced, and when they are again arrested, after 

 the last apparent transformation, there are only eight 

 ganglia to be seen. The second, third, fourth, and 

 fifth have given rise, by their fusion, to two large 

 masses, which are placed in the chest quite close to 

 each other; the sixth and seventh have completely 

 disappeared, and their former position is alone marked 

 by the origin of a few nervous filaments; the five 

 posterior ones have undergone little or no change. 

 Finally, the brain itself has become twice as large, • 

 its lobes have assumed a transverse position, and each 

 of them gives origin to a large optic nerve which 

 travels to one of the compound eyes.* 



* This short sketch of the changes undergone by the nervous 

 system has been taken in great part from the researches of Herold, 

 because his investigations refer to the same insect (Pieris Brassicse) 

 which Reaumur studied. On which account we have been enabled 



