INSECTA. 203 



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

 of some Coleoptera with very rudiment.al feet, have a simple ven- 

 tral nervous chord, almost as devoid of ganglionic enlargements as 

 in the Nematoideaand lulidse: 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. 

 104, 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 

 that they found in the works of Lyonnet corroboration of this in- 

 conclusive physiological view. Lyonnet, however, expressly denies that 

 the parts which he called brain and spinal marrow in the insect were 

 similar in anatomical structure to those in the higher animals. 



" The spinal marrow of the caterpillar, if one may say that it pos- 

 sesses such," observes Lyonnet, " sensibly differs from that of man. 

 It is slender ; it bifurcates at intervals, and enlarges from distance 

 to distance to form its masses, which I have named ganglions." 

 The intervening chords Lyonnet terms " conduits de la Moelle 

 ^piniere." He particularly points out the difference in relative posi- 

 tion, and in the means of protection assigned to the ganglionic 

 columns in insects, and to the spinal chord in the higher animals. 

 As to any views of distinct physiological properties in the ganglions 

 or the non-ganglionic nervous tracts, none such appear in the 

 works of Lyonnet ; nor, indeed, did they form part of the do- 



