NERVOUS SYSTEM. INVERTEBRATA. 35 



to form a common neuropile- stalk (peduncle, G). This 

 stalk penetrates directly downwards through the substance 

 of the protocerebral lobe till it reaches the mid-horizontal 

 plane of the brain. Here it gives off two branches, 

 one of which (cauliculus, H) curves forward and upward 

 near the anterior surface of the brain to terminate in a 

 rounded end close beneath the anterior lip of the outer 

 calyx ; the other (trabecula, I) runs diagonally downwards 

 and inwards, till it meets its fellow, without fusion, in 

 the mid-line. The fungiform bodies do not directly give 

 rise to any nerves, but they enter into close relations with 

 all parts of the brain by means of fibrous tracts, the most 

 important of which pass to the optic ganglion of the same 

 side, to the opposite fungiform body, to the corpus centrale, 

 and to the antennary lobe of the opposite side. Fibres 

 belonging to this latter tract continue directly past the 

 fungiform body into the optic- ganglion, and constitute an 

 optico-olfactory chiasma similar to that seen in Decapod 

 Crustacea. 



In spite of the evident importance of these structures 

 their function is still unknown, but it is noteworthy that 

 within the same Order their size increases roughly in pro- 

 portion to the intelligence of the Insect, and among social 

 forms they may even vary in development between the 

 persons of the society being, for instance, proportionately 

 larger in the Worker bee than in the Drone or Queen. 



The protocerebral lobes are united across the mid-line 

 by two bands of deeply staining neuropile. One of these 

 (the pons) lies immediately below the dorsal point of union 

 of the two halves of the brain ; it is in the form of a for- 

 wardly directed horseshoe, and stands in close relation to 

 the roots of the nerves to the white spots (vestigeal ocelli). 

 A similar though backwardly directed horseshoe-shaped 

 band gives origin to the ocellary nerves v in Limulus, and 

 there is a somewhat similar strand in the fore part of the 

 brain in Decapod Crustacea. 



The second commissural band (corpus centrale) is larger 

 and of more complicated structure ; it has the form of a 

 crescentic plate hollowed below, and consists of two layers 

 of neuropile. It lies approximately in the centre of the 



D 2 



