EUPAGUEUS. 531 



mass. A slender transverse nerve — the post-oesophageal 

 connective — joins the two commissures a short distance 

 behind the oesophagus. 



The Thoracic Ganglion-mass (fig. 37), in which the 

 oesophageal connectives terminate, lies over the sternal 

 artery on the endosternal plates of the third to sixth 

 thoracic somites. Inward projecting processes from the 

 dorsal parts of the endosternites partly bridge over the 

 valley in which it is found. It is composed of three large 

 masses of fused ganglia, which are divided from each 

 other by constrictions; the first and second of these are 

 somewhat rectangular in shape, and the third is 

 pyriform. A shallow semi-circular groove on the third 

 division marks the separation of the ganglia which are 

 the centres for the fourth and fifth pereiopods from those 

 which belong to the first abdominal segment. 



The Central Thoracic Nerve-mass is pierced in the 

 mid-line in three places for the passage of arteries. The 

 most posterior of the three is a huge foramen, through 

 which the descending portion of the sternal artery passes ; 

 the others are small perforations, which are almost 

 indistinguishable in any but specimens injected for the 

 blood system (asc. a.). 



Seven paired ganglia take part in the formation of 

 the anterior mass, and seven nerves radiate outwards 

 from it on either side. Those supplying the mouth 

 parts usually come off separately, but there is consider- 

 able variation in their arrangement. The branches from 

 the first and second Maxillipedes often coalesce before 

 joining the main nerve trunk, and, less frequently, 

 fusion takes place between the nerves supplying the pairs 

 of Maxillae. The thoracic ganglion mass is symmetrical 

 throughout, so only one side need be described. 



The First nerve originates close to the oesophageal 



