208 Descriptio7i of the Plates. 



it between the anterior gastric muscles, from which, as also 

 from its posterior prolongation, nerves are given off down- 

 wards on either side of the stomach. 



q. Azygos nerve, passing downwards from the middle of the 

 posterior edge of the supra-oesophageal mass to meet two 

 pairs of nerves given off from the thickenings developed 

 upon the commissural cords of the nerve collar as they cross 

 the oesophagus, and form with them the compound nervus 

 recurrens, q. The forward position of the supra-oesophageal 

 ganglionic mass would appear to render it impossible for the 

 Crayfish to have any ganglion frontale developed upon 

 such as Insects possess. Leydig, however, appears to have 

 discovered such a ganglion in the Oniscus. This nerve, /, 

 seems to correspond with the nerve-cord passing backwards 

 from the ganglion frontale in Insects before it receives any 

 branches of communication from the paired ganglia; the 

 compound nerve, q, would then correspond to the similarly 

 compounded nerve of Insects, upon which ganglia are 

 frequently developed successively from before backwards 

 in relation with the digestive tract, whilst the ganglion 

 frontale must be considered to have coalesced with the 

 supra-oesophageal mass. 



r. Fifth abdominal, or sixth post-oral ganglion. It is more 

 closely approximated to the ganglion next in front of it 

 than any of the ganglia either in front of or behind it are 

 to each other. Posteriorly to it are the six post-abdominal 

 ganglia. From this ganglion and the two in front of it, 

 long nerves pass off upwards to the reproductive organs and 

 the superiorly placed muscles. 



s. Multi-articulate flagellum of inferior or outer pair of antennae, 

 the ' antennae,' strictly so called, supported by a peduncle 

 consisting of five joints, to which the scaphocerite and a 

 smaller calcified nodule are laterally articulated. 



t. Paired flagella of upper pair of antennae, or antennules, carried 

 by a triarticulate peduncle, as are the antennae of Orthop- 

 tera. On the larger of these two flagella certain delicate 

 membranous cilia are to be found, which, though of very 

 various forms, are yet constant in Crustacea, and are sup- 

 posed to be olfactory organs by many authors. 



