Common Crayfish. 107 



ganglia. This ganglion has been said to represent two ganglia, 

 which were separate and distinct in the embryo. Rathke^ however, 

 informs us, 1. c, that he has not been able to observe anything as 

 to the mode of origination of the post-abdominal ganglia ; and 

 though the distribution of the nerves of the sixth ganglion may 

 seem to make it probable that it was so developed, the comparative 

 anatomy of the homologous series in other Crustacea would appear 

 to militate against such a view. A system of nerves homologous 

 with the nervi transverd or * brides epinieres' of Lyonet, described 

 under Prep. 29, p. 84, in the insect, is represented by a nerve 

 which arises in each segment, as seen with the naked eye in the 

 third post-abdominal segment over the slip of blue paper placed 

 there, from the commissural cord, and bifurcates into two lateral 

 nerves. The bilaterally symmetrical and the azygos stomato- 

 gastric system of insects are represented in the Crayfish by a 

 system of nerves which arises symmetrically on either side of the 

 oesophagus from a thickening on the commissures of the nerve 

 collar, and by an azygos nerve which arises from the supra-oeso- 

 phageal mass ; but which has not any separate prae-cerebroid gan- 

 glion frontale developed upon it as in insects. A slip of blue paper 

 has been placed between the commissural cords of the nerve collar, 

 and the antennary sternum with which they are commensurate 

 in length ; and in the interval between the commissural cords, the 

 various trunks of the stomato-gastric system are seen. The com- 

 missural cords are connected anteriorly to the first post-oral gan- 

 glion by a transverse cord, which is in apposition with the posterior 

 wall of the oesophagus, and represents, probably, an anterior frag- 

 ment of the backwardly displaced ganglionic mass which supplies 

 the jaws and foot-jaws. The prae-oral ganglionic mass, which in 

 embryonic life was seen by Rathke to be made up of two ganglia 

 on each side, of which the posterior or the one more nearly placed 

 to the mouth was the larger, and supplied the antennae and anten- 

 nules, sends nerves to these organs, to the auditory organ lodged 

 in the antennule, to the antennary gland, and finally to the eyes. 

 The ocular portion of the prae-oral mass has not assumed the 

 degree of relative importance which it has in air-breathing Ar- 

 thropoda. 



The distinctness of the fibrous chords from, and their position 

 superiorly to the ganglionic elements of the ventral chain, can 



