INSECTA. 359 



an ovum, — there and then undergoing that part of their development 

 which before was left incomplete : finally they emerge in their perfect 

 state to enjoy for a brief period the highest faculties, animal and 

 organic, which they are destined to acquire ; fluttering in the air, it 

 may be, for a single day, procreating their kind, and perishing. Now 

 the development of the nervous system, like that of the muscular, 

 digestive, and other systems, being completed at distinct and some- 

 times remote periods, requires to be studied in the first and last of the 

 active states of the insect, and also in the intermediate period, when, 

 owino; to the rapidity and extent of the changes which it undergoes, 

 the nervous system offers to the comparative anatomist and physiologist 

 phenomena of the highest interest. 



The apodal entozoiform larvie, in which the segments of the body 

 are obscurely defined, as those of most Diptera, Hymenoptera, and 

 of some Coleoptera with very rudimental feet, have a simple ventral 

 nervous chord, almost as devoid of ganglionic enlargements as in 

 the Nematoidea and lulidge: it is, however, usually relatively shorter, 

 failing to reach the posterior extremity of the body, and the fine nerves 

 pass off on each side and radiate from the extremity. 



In the larva of Stratiomys chamceleon the ventral chord is divided 

 by a series of constrictions into eleven consecutive and contiguous 

 ganglia. 



The larvae, which present, like the Centipede, larger and more 

 definite segments, most of which are provided with legs or prolegs, 

 have a ganglionic centre for each segment, and intermediate chords. 



This anellidous and chilopodiform type of the nervous system has 

 been best described and figured by Lyonnet.* The subject which this 

 inimitable dissector and artist selected for his patient investigations 

 was the caterpillar of the Cossus ligniperda. The nervous axis 

 consists of thirteen ganglions, arranged along the median line of the 

 body, and connected by two chords or columns. The first and largest 

 ganglion, situated in the head above the mouth, and of a bilobed form, 

 Lyonnet calls the brain ; the remaining twelve ganglions (as in Jig. 

 159, 1 to 12.) are situated below the alimentary canal; the eleventh 

 and twelfth are so close together that their distinction might readily be 

 overlooked ; but it was pointed out by Lyonnet. The sub-abdominal 

 ganglions and inter-communicating chords were called by Lyonnet 

 the spinal marrow. Some anatomists who have applied the analogy 

 of the ganglionic and non-ganglionic roots of the spinal nerves in the 

 higher Vertebrata to the explanation of the functions of the ganglionic 

 and non-ganglionic parts of the nervous axis in Insects, have thought 



* CCXL. 



A A 4 



