NO. 4 NERVOUS SYSTEM OF A CENTIPEDE — LORENZO 5 



Typically there is a pair of ganglia for each body segment but, in 

 most of the arthropods, the pair is fused. The ganglia are intercon- 

 nected by longitudinal and transverse connectives and commissures 

 respectively. In the higher insects the ganglia of adjacent segments 

 are so concentrated as to be indistinguishable as individual entities. 

 This coalescence reaches an extreme in the Diptera, where the single 

 thoracic ganglion is in reality an amalgamation of many segmental 

 ganglia. 



Transverse sections reveal the typical ganglion as being composed 

 of an inner core of nerve fibers, termed the neuropile, and an outer 

 cortex of neurons. This arrangement is the reverse of that found in 

 the vertebrate spinal cord, where the fibers are located in the cortical 

 white matter, and the nerve cells in the medullary gray matter. Since 

 sensory neurons are situated in the epidermis, the neurons contained 

 within the central nervous system are either motor or internuncial in 

 nature, and are typically unipolar. Multipolar and bipolar cells are 

 found in the vicinity of the receptors. Within the fibrous neuropile 

 are found varying amounts of neuroglial cells. 



The central nervous system consists of a brain, or supraesophageal 

 ganglion, the subesophageal ganglion, and the ventral nerve cord. The 

 brain is the dorsal aggregation of nerve tissue in the cephalic capsule 

 situated either anterior or dorsal to the esophagus, depending on the 

 shape of the head. According to the terminology of Viallanes (1887), 

 the brain is divided anatomically into three regions known as the pro- 

 tocerebrum, deutocerebrum, and tritocerebrum. The regions, however, 

 are not always clearly defined externally. In most insects the greater 

 portion of the protocerebrum is associated with the visual apparatus. 

 Where eyes are absent or poorly developed, this region is reduced." 

 The protocerebrum is further divided into protocerebral lobes and 

 optic lobes. The second region of the brain, the deutocerebrum, in- 

 nervates the antennae. For the most part, it is sensory and internun- 

 cial in function but may contain some motor elements associated with 

 the antennal musculature. Efferent fibers may emerge from the an- 

 tennal lobes independent of the sensory roots and are sometimes called 

 accessory antennal nerves. The third region, the tritocerebrum, is 

 represented as the ventral portion which communicates directly with 

 the subesophageal ganglion by way of the circumesophageal con- 

 nectives. It is through the tritocerebrum that the central nervous sys- 



6 Power (1946a) has found that, although quantitative differences exist, there 

 are no detectable qualitative architectural modifications in the brain of eyeless 

 Drosophila mutants. There is no correlation between the inability to see with 

 a qualitative hypoplasia of the central nervous system. 



