308 DESCRIPTION OF THE PLATES. 



p. 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 other Arthropoda it is generally fused with more or 

 fewer of them. 



q. Median stomatogastric nerve formed by the fusion of four nerves, two 

 from each oesophageal ganglion, to q'. 



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

 edge of the supra-oesophageal mass to meet four nerves given off, 

 two from each of the two oesophageal ganglia, developed upon the 

 commissural cords of the nerve-collar. It forms with them the 

 nerve, q. The nerve from the anterior edge of the supra-oesophageal 

 ganglion is not shown. Cf. p. 189, ante. 



r. Fifth thoracic, or sixth post-oral ganglion. Posteriorly to it are the 

 six 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 or endopodite of second pair of antennae. 



/. Exo- and endopodite of first pair of antennae or antennules. 



u. Second joint of third thoracic appendage, ' maxilliped,' or 'foot-jaw.' 

 This joint, as in the forceps v I, next behind it, represents two joints, 

 the basipodite and the ischiopodite of the normal seven-jointed endo- 

 podite, seen in v 2, v 3, v 4, v 5. Its antero-internal edge is denticu- 

 late, as in the Lobster, but its serratures are concealed by a fringe of 

 setae. 



v i. Fourth thoracic appendage, or forceps, modified by the production of 

 the distal outer angle of the propodite so as to form a pair of pincers 

 with the opposed last joint, or dactylopodite. Two other joints, the 

 ' carpopodite ' and ' meropodite,' are shown in this figure. 



v 2 and v 3. Fifth and sixth pairs of thoracic or first and second ambu- 

 latory legs, differing from v i in their smaller size, and in not having 

 the second and third joints fused. 



v 4 and v 5. Seventh and eighth pairs of thoracic or third and fourth 

 ambulatory legs of the Decapod. The two terminal joints do not 

 form pincers, otherwise they resemble v 2 and v 3. The vas deferens 

 opens in the coxopodite or basal joint of v 5. 



w i and w z. The appendages of the two first abdominal somites modified 

 so as to form an accessory copulatory organ in this a male specimen. 



iv 3, w 4, and w 5. Appendages of the third, fourth, and fifth abdominal 

 somites, consisting each of two basal joints, which serve as a pedicle 

 to two multiarticulate filaments, an exopodite and an endopodite. 



