NERVOUS SYSTEM 473 



The central body is well marked and occupies a typical position. 



The optic lobes each contain two optic masses, the lamina and the medulla. From 

 the latter a well-marked band of fibres passes backwards and then sharply inwards to 

 the protocerebral bridge. 



The protocerebral bridge consists of four glomerular masses. 



The corpora pedunculata appear to be represented on each side by a small but 

 distinct glomerular mass in the roof of the protocerebrum on a level with the proto- 

 cerebral bridge. From this mass a band of fibres passes inwards and, looping round 

 the optic tract, extends downwards to the outer edges of the central body. Here some 

 fibres enter the latter. Others cross over and join the corresponding tract on the other 

 side, and the remainder continue downwards and outwards to the antennular}- nucleus 

 to end in a glomerular mass which may be distinct or may be a dorsal lobe of the latter. 



The antennulary nucleus is a glomerular structure occurring in the antennulary lobe. 

 It consists certainly of two main lobes, an inner and an outer, the latter on each side 

 being connected by a broad commissure — the main antennulary commissure. Certain 

 fibres of the antennulary nerve pass directly between the two lobes and extend down the 

 inner side of the nerve ring to the tritocerebrum, joining a thick bundle which emerges 

 from the posterior lobe. 



The nucleus of the nauplius eye consists of three glomerular masses embedded in 

 the median mass of nerve cells formed from the fused optic lobes. The three nerves 

 of the nauplius eye each connect with one of these glomeruli. Posteriorly each glomerulus 

 gives rise to a tract of fibres which pass back between the constituents of the proto- 

 cerebral bridge to the central body, and finally to the main antennulary commissure. 

 In the latter, as well as in the central body, each tract gives off fibres to both right 

 and left sides of these structures. 



In front of the nucleus of the nauplius eye, between the nerves emerging from it, is 

 a group of giant cells from which emerge the two nerve fibres to the frontal organ. 

 Holmgren (1916, p. 118) has described a similar arrangement in Apiis. 



There is a large commissure immediately in front of the central body, as well as a 

 small one below the protocerebral bridge. 



The position of the nuclei of the segmental nerves can be seen from Fig. 9. It will 

 be noted that there is no distinct nucleus for the antennal nerve. The main bulk of the 

 fibres of the lower antennal nerve pass inwards and then curve slightly towards the 

 antennulary nucleus, but they form no compact nucleus. The region in which they 

 terminate is a tangled mass of medium-sized giant fibres. The remainder of the fibres, 

 apart from the giant fibres, pass backwards along the inside of the nerve ring to the 

 tritocerebral commissure. 



Although the third maxillary nerve does not enter the nerve ring direct, its fibres 

 apparently pass through the stalk into the hinder part of the nerve ring, where they 

 terminate in a nucleus which abuts against the nucleus of the first and second maxillary 

 nerves. These nuclei are joined by a distinct commissure. 



The sympathetic system has its own nucleus just in front of the mandibular nucleus. 



