Common Crayfish. 207 



Brachyiira open into the upper part of the pyloric part of 

 the stomach, just posteriorly to its valvular apparatus, and 

 close to its opening- into the duodenum. In Brachyura there 

 is a third sac homologous with the single sac observable in 

 some, and the two sacs observable in other Amphipoda as 

 opening into the duodenum just before its junction with the 

 rectum, and sometimes called a 'renal organ/ It is not 

 found in the Fresh-water Crayfish, though it is in the 

 Lobster. 



0. Supra-oesophageal gang-lionic mass, immediately posteriorly to 

 the scaphocerite or squamiform exopodite of the inferior or 

 externally placed pair of antennae, the 'antennae^ properly 

 so called. The likeness which this 'scale' of the antennae 

 bears to the exopodite of the sixth abdominal segment is, as 

 Fritz Miiller has remarked, curiously illustrated by the fact 

 that its ordinary function of lodging the auditory organ is 

 sometimes, as in Mijsis, transferred from it to that appendage. 

 Both facts find their explanation in the view which regards 

 both segments and both sets of appendages as belonging to 

 a 'primitive body' corresponding with that which the 

 Naupliiform larvae of Cirripedia, Copepoda, and Phyllopoda 

 bring with them out of the q^^. The greater relative size 

 of the scaphocerite is one of the external points of dijQference 

 between the Crayfish and the common Lobster. 



2i. First post-oral ganglion, supplying the mandibles, the two 

 pairs of maxillae, and the three pairs of foot-jaws, or thoracic 

 appendages. In the developing Astacus this mass consists 

 of six pairs of ganglia, in correspondence with the six sets 

 of appendages it innervates. In Insects, the first post-oral 

 ganglion is always distinct from the thoracic ganglia, whilst 

 in all other Arthropoda it is fused with more or fewer of them. 



q. Nervus recurrens, formed by the junction to an azygos nerve, 

 homologous to the nervus recurrens of Insects, and repre- 

 sented by the single trunk (f , of two pairs of nerves, which 

 arise from the nerve-collar on either side of the oesophagus. 

 This compound nervus recurrens of the Crayfish passes up 

 the anterior face of the oesophagus and stomach, and on 

 the angle formed by the junction of this anterior with the 

 dorsal wall of the organ, it has a ganglion developed upon 



