NERVOUS SYSTEM. TNVERTEBRATA. 19 



condition, and reminds one, in the ladder-like construc- 

 tion of its ventral chain, of that of certain Tube-worms 

 (e. g. Serpula). The cerebral ganglion is a small quadri- 

 lateral body, situated in front of the oesophagus close 

 beneath the eyes, and so placed that its proper ventral 

 surface faces upwards and its anterior border backwards. 

 It gives off a pair of nerves from the outer angles of its 

 true anterior end to the eyes. The first pair of antennae 

 are supplied by a pair of small nerves that seem to rise 

 from the circumoesophageal connectives ; their true centres 

 of origin are, however, situated in the lateral parts of the 

 cerebral ganglion. On a level with the posterior margin 

 of the mouth, each circumoesophageal connective enlarges 

 to form an oesophageal ganglion, which gives off two nerves 

 a small one from its outer aspect to the second antenna, 

 and a visceral nerve from its inner surface ; the two ganglia 

 are united by a double commissure. The condition of the 

 antennary nerves in Apus suggests that the direct origin 

 of these nerves in the higher Crustacea from the cerebral 

 ganglion is the result of an anterior concentration of 

 centres originally separate and post-oral in position. The 

 ganglia of the ventral chain are paired and, in the anterior 

 region of the body, widely separate. They are united to 

 one another transversely by a double commissure and longi- 

 tudinally by a pair of connectives. In the posterior part 

 of the body the ladder-like appearance of the chain is lost 

 owing to the shortening of the commissures and connectives, 

 although the individuality of the ganglia is maintained. 

 The ganglia correspond in number and position to the 

 appendages, and are thus more numerous than the body 

 segments. The second pair of maxillse alone seem to have 

 no corresponding ganglia ; their nerves arise from the 

 connectives. The specimen is so small that only the most 

 important of the above-mentioned characters are visible, 

 such as the wide lateral separation of the ventral-chain 

 ganglia and their gradual approximation to one another 

 posteriorly. 0. C. 1302 L. 



Pelseneer, Quart. Journ. Micro. Sci., vol. xxv. 1885, 

 p. 433. 



