470 DISCOVERY REPORTS 



ganglion, and pass out as a bundle which can be traced to the tip of the limb. In the 

 angle between these two bundles of fibres there is a group of four or five bipolar cells, 

 from which arise medium-sized giant fibres (Fig. lo A, see later, p. 474). 



The two roots of the antennal nerve fuse into a basal ganglion, more or less conical 

 in shape, lying between the powerful muscles of the protopodite. The ganglion lies 

 immediately in front of the antennal gland and is embedded in the peculiar tissue, an 

 extension of the gut parenchyma, which I have already described. 



The upper root of the antennal nerve consists of a bundle of five giant fibres. These 

 pass directly through the basal ganglion (Fig. 11) and will be described in a later 

 section. The lower root contains one giant fibre which also passes through, but its 

 main mass consists of minute fibres which, on entering the ganglion, divide into four 

 main tracts. These pass through the ganglion and emerge as radiating nerves which 

 can be traced to the superficial parts of the limb. A minute nerve can be traced from 

 the ganglion to the duct of the antennal gland (Fig. 11). 



The mandibular basal ganglion is a compact ovoid mass occupying the proximal half 

 of the basipodite. A tract of fibres passes through it, and bifurcates into a small branch 

 running to the exopodite and a larger branch which can be traced to the tip of the endo- 

 podite. 



The maxillulary basal ganglion is less compact than the mandibular ganglion. It is 

 quite distinct in its proximal portion, but it tapers off distally into a string of nerve 

 cells closely associated with large gland cells (Fig. 10 B). 



In the maxilla there is no single basal ganglion, but rather a group of distinct ganglia 

 connected by a network of anastomosing nerve cells. The second maxillary nerve passes 

 down the front of the limb and, after giving off a small branch, extends as a chain of 

 nerve cells to an elongated ganglion b which reaches the lower edge of the first joint. 

 The first maxillary nerve passes to a compact ganglion a at the hinder margin of the 

 first joint. From the lower corner of this ganglion a connective passes to a small sub- 

 spherical ganglion c, lying just inside ganglion h. The second maxillary nerve, soon 

 after entering the limb, gives off a thin branch which, after sending a connection to 

 ganglion o, terminates in a plexus of nerve cells lying on the median side of ganglion c. 



The first trunk limbs contain a conspicuous mass of gland cells, on the anterior face 

 of which occurs a layer of nerve cells forming the basal ganglia of these limbs. 



The basal ganglion of the second trunk limb — the brush limb — consists of a compact 

 ovoid mass situated at the base of the limb. 



The whole system of basal ganglia may be compared with the parapodial ganglia 

 of Polychaetes or the ganglia at the base of the appendages of Tardigrades (Marcus, 

 1928). However, Dr Hanstrom has suggested to me that, strictly, they should not 

 be called ganglia until it has been shown that they contain ganglion cells. The basal 

 ganglion of the antennule has always been termed a ganglion, and we must use the term 

 merely for convenience but, until the constitution of these basal ganglia has been studied 

 by special neurological methods, the possibility remains that they are nothing more than 

 accumulations of sensory cells which have passed in from their primitive superficial 



