4 6f, THE NERVOUS SYSTEM. 



tliis nerve-root (PI. XXIX., Fig. 21. There is a distinct analogy 

 between these decussating tracts and the anterior pyramids 

 of the medulla oblongata of Vertebrates. 



The Maxillary Nerves. This pair of nerves arise from the 

 lowest part of the infra-cesophageal ganglia. They are the 

 great nerves of the proboscis, and supply all its structures 

 except the pharynx. The fact that only a single pair of nerves 

 supplies the proboscis is in favour of the view that it consists of 

 only a single pair of metameral appendages. The question 

 may arise : why should these nerves be termed maxillary 

 rather than labial ? 



In answer it may be remarked that there is a tendency to- 

 wards the reduction of the size of the labial, and an increase 

 in that of the maxillary nerves, as we advance from the less to 

 the more highly-specialised Insecta. I know no insects which 

 exhibit the reverse condition. This would not, perhaps, be a 

 cogent argument taken by itself, but it is worthy of note in 

 relation to my contention that the proboscis is developed from 

 the maxillae of the embryo. 



The Ganglia of the Crura (PI. XXX., Fig 3, ^ cr). These con- 

 sist of two distinct lobes in front of the crura, which are 

 covered by a thick layer of nerve cells, occupying the sulcus 

 between the antennal and infra-cesophageal ganglia. They give 

 origin to the pharyngeal nerves, and probably represent the 

 ganglia of the antenna of the Crayfish, from which the phann- 

 geal and antennal nerves arise in close proximity to each other. 

 Gaskell has compared the ganglion of the crus with the locus 

 niger of the Vertebrate, but it appears to me that evidence is 

 wanting to justify such a comparison. 



The Pharyngeal Nerves arise from the crura between the infra- 

 cesophageal and the antennal ganglia by two roots, and from 

 the large superficial cells of the ganglion of the crus, which lie 

 in the sulcus between the antennal and infra-cesophageal 

 centres. These nerves are far smaller than the maxillary, and 

 are distributed to the pharyngeal muscles. 



The Pyramidal Ganglion. This is a large pyramidal group of 

 nerve cells, many of which are distinctly stellate. It is 



