V CRUSTACEA— NERVOUS SYSTEM 349 



In others the number is evidently reduced by the fusing of originally separate 

 ganglia. In Qnaihophausia we find besides the infra-oesophageal ganglion (which 

 most probably consists of the fused mandibular and maxillary ganglia) 8, in Eucopia 

 however only 6, thoracic ganglia. The most anterior thoracic ganglion has in this 

 latter case probably united with the infra-cesophageal ganglion, and the last thoracic 

 ganglion with the last but one. This is made probable by the fact that in the last 

 thoracic segment no ganglion occurs. In Mysis [relicta) the thoracic ganglia are 

 even said to be fused into a longitudinal strand. The a3sophageal commissures are in 

 many Schizopoda {Euphausia, Boreomysis) connected directly behind the cesophatnis 

 and in front of the most anterior ventral ganglion by a transverse commissure, which 

 perhaps corresponds with the commissure of the ganglia of the posterior antennse often 

 mentioned above. 



All the Schizopoda seem to possess 6 abdominal ganglia. The 

 nerves which supply the body musculature of the abdomen diverge 

 from the longitudinal commissures half-way between 2 consecu- 

 tive ganglia. This arrangement seems to be characteristic of all 

 Tlioracostraca. 



In the Cumacea {Diadylis) the ventral chord consists of 16 pairs of 

 ganglia ; the three most anterior of these, which have moved near each 

 other, supply the oral limbs ; 7 thoracic, and 6 abdominal pairs of 

 ganglia follow. 



The nervous system of the Stomatopoda in its segmentation shows 

 very clearly a close relation to the metamerism of the body. In the 

 cephalo-thoracic portion of the ventral chord only the 3 ganglia of the 

 3 most posterior thoracic segments have remained separate, i.e. of 

 those segments which, uncovered by the cephalo-thoracic shield, 

 carry the biramose ambulatory feet. All the other preceding ganglia 

 are united into a large infra-oesophageal ganglion. The oesophageal 

 commissures are very long, and show behind the oesophagus the 

 transverse commissures often mentioned above. The 3 posterior 

 thoracic ganglia are followed by the 6 abdominal ganglia which are 

 characteristic of the Thm-acostraca. 



Deeapoda. — We here find many grades of concentration, from the 

 still tolerably richly segmented nervous system of the Macriira to the 

 nervous system of the Brachyura, in which all the ganglia of the 

 ventral chord have fused into one single thoracic ganglionic mass. 

 Taking as a type of the Macrura the Cray-fish, Astacus fluviatilis (Fig. 

 •236, B), the highly developed brain gives off the nerves for the eyes, the 

 anterior antennae, and from its posterior part for the posterior antennae. 

 The oesophageal commissures are of considerable length and connected 

 behind the oesophagus by a transverse commissure. In the course of 

 each oesophageal commissure lies a small ganglion, the so-called com- 

 missural ganglion. From these 2 commissural ganglia various nerves 

 diverge, among which the visceral nerves and the nerves of the 

 mandibles are to be specially noted. The latter indeed have their roots 

 in the infra-oesophageal ganglion, but are united with the oesophageal 

 commissures as far as the commissural ganglia. The oesophageal 

 commissures enter an infra-oesophageal ganglionic mass which consists 



