COMMON CRAYFISH. 



173 



supplying the eyes. The natural conclusion is that the supra-oesophageal ganglion 

 consists of two distinct parts in Apus, &c., i. e. of a true prae-oral ganglion supply- 

 ing the eyes, and a second pair of ganglia shifted forwards supplying the first 

 antennae. The shift forwards explains the backward course of the nerves. Whilst 

 in Apus, &c., the second antennae have a post-orally placed ganglion, in higher 

 Crustacea this ganglion also has shifted forwards ; but there is evidence to show 

 (embryo Astacus) that there is a distinct ganglion, supplying in this instance both 

 first and second antennae, which fuses with an anterior rudiment to form the supra- 

 oesophageal ganglion of the adult. It seems probable therefore that the first 

 antennae, like 'the second, are in reality primitively post-oral appendages, or at any 

 rate are homologous with the limbs borne by the post-oral somites of the body. 



The Arachnida afford us an instance of a group of Arthropoda in which all the 

 appendages are embryonically post-oral. The first pair of appendages however is 

 invariably shifted in front of the mouth during growth. In Scorpio and Limulus at 

 least their nerves are said not to come from the supra-oesophageal mass but from 

 the commissures. The ganglia however have not been investigated microscopically. 

 It is possible that these animals retain an archi-cerebrum, i. e. a supra-oesophageal 

 ganglion not fused with other ganglia placed posteriorly to it. The Crustacea, on 

 the contrary, evidently possess a syncerebrum, i. e. a supra-oesophageal ganglion 

 fused with one or two posteriorly placed ganglia. 



The following table shows in parallel columns the post-oral somites and their 

 appendages in the Crustacea, Arachnida, Myriapoda, and Insecta. The antennae 

 of the two latter classes are not included as being apparently processes of the pro- 

 cephalic lobes. The post-oral appendages of Crustacea and Arachnida which 

 become prae-oral are printed in a different type, and the first antennae of the former 

 Class are marked with a note of interrogation as being somewhat doubtfully homo- 

 logous with the succeeding appendages. 



See, for remarks on the descent of Arthropoda, Balfour, Comparative Embry- 

 ology, i. p. 45 1 ; for a discussion on the relations of the Arachnida and Crustacea, 

 Kingsley, Notes on the Embryology of Limulus, Q. J. M. xxv. 1885, p. 556. 



TABLE OF POST-ORAL SOMITES AND THEIR APPENDAGES 

 IN ARTHROPODA. 



