ANURIDA. 391 



maxillulse and the first and second maxillae, together with 

 an unpaired nerve Avhich passes to the lingua. 



The prothoracic and mesothoracic ganglia each give 

 off a pair of principal nerves on either side, which supply 

 the various muscles of their respective segments. The 

 ganglion situated in the metathorax (th. g. 3) is in reality 

 a ganglionic complex formed by the fusion of the pair of 

 primitive metathoracic ganglia with those of the 

 abdomen. Several pairs of nerves arise from this ganglion, 

 and they appear to supply the musculature of the meta- 

 thoracic and first abdominal segments. 



Directed backwards into the abdomen is a pair of 

 stout parallel nerve cords (nv.). These are, perhaps, to be 

 regarded as the remaining connectives of the abdominal 

 nerve chain, the nerve ganglia of which have migrated 

 forwards during embryonic development and become 

 fused with the metathoracic ganglion. Along their course 

 these nerves give oft' branches (l.n.) which can be traced to 

 the generative organs and to the musculature. 



Histologically, the nervous system is constructed 

 upon an exceedingly simple plan. The brain is composed 

 on the outside of a layer of unipolar and bipolar ganglion 

 cells provided with very small but deeply-staining nuclei. 

 Internally it consists of a mass of extremely fine nerve 

 fibres, which are arranged for the most part in transverse 

 and longitudinal directions. It presents none of the 

 complex arrangement of nerve cells and fibres which is 

 exhibited in the brain of most Pterygote Insects. The 

 succeeding ganglia also each consist of an outer layer of 

 nerve cells enclosing a central core of nerve fibres (figs. 46 

 and 47). Nerve cells are wanting from the thoracic 

 connectives, but a few are distributed along the course of 

 the abdominal cords. All the ganglia and connectives 

 are invested by a neurilemma (near, in figs. 46 and 47) of 



