138 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



mile, then turn anteriorly and run the length of 

 the antennule including the flagella. This nerve 

 is probably mixed, since it sends off branches to 

 the muscles as well as to the sensory flagella. 



Except for the work of Keim (1915) on the 

 nerves of Astacm, little information exists on the 

 innervation of the crustacean antennule. The sit- 

 uation in Penaeus appears to be roughly compa- 

 rable to the arrangement of the antennular nerves 

 in Astacus. The most striking difference is the 

 size of the nerve innervating the statocyst. That 

 of Penaeus is very much larger than the statocyst 

 nerve of Astacus. 



ANTENNAL NERVES 



Figures 24, 75, 76 



The antennal nerves pass out of the supraeso- 

 phageal ganglion posteroventrally, from a point 

 laterad of the circumesophageal connective. The 

 antennal nerves proceed ventrocaudally for a short 

 distance and then turn anteriorly to run into the 

 antenna through its large foramen, or into the 

 mass of the antennal muscles. Within the body 

 of the antenna, the nerves give off branches to 

 various muscles and sensory endings in the anten- 

 na and scale. One branch enters the antennal 

 flagellum (fig. 24, B) and is extensively subdi- 

 vided. Small groups of neurons are thus split 

 off to each of the many sensory elements in the 

 flagellum (fig. 25). 



The antennal nerves of Penaeus appear to be 

 similar to those described by Keim in Astacus. 



NERVE TO PROTOCEPHALON ATTRACTOR 

 MUSCLE 



Figures 75, 76 



This structure comes out of the dorsal brain 

 between the antennal nerves and the circumeso- 

 phageal connective and passes into the substance 

 of the protocephalon attractor muscle. I have 

 previously offered objections to assigning the pro- 

 tocephalon attractors to the "antennal" segment, 

 on grounds of multiple insertions of the muscle. 

 However, the position of the protocephalon at- 

 tractor nerve origin on the brain tends to suggest 

 that at least part of the muscle has affinities with 

 the antenna. 



LABRAL NERVE 

 Figures 27, 76 



Each lateral half of the superficially unpaired 

 labrum is supplied with a labral nerve descending 



from the tritocerebral ganglia (illustrated as the 

 labral ganglion in fig. 27). Upon entering the 

 labral sac, the nerve divides into parts which go 

 to many labral muscles, to glands, and to sensory 

 endings in the epidermis. From the work of 

 Schmidt and Keim on the muscles and nerves jdi 

 Astacus one receives the impression that this 

 European crawfish does not posses a labrum, since 

 no muscles or nerves are listed, nor in fact is the 

 labrum mentioned. Henry (1948a) finds that the 

 labral nerve of Pahtemon paucidem de Haan and 

 other Crustacea arises from the region of the pri- 

 mary stomodaeal ganglion, a stomatogastric gan- 

 glion connected to the tritocerebrum. 



TRITOCEREBRAL GANGLIA AND NERVES 



The tritocerebral ganglia or lobes represent the 

 ganglia of the first ventral segment. As has been 

 previously mentioned, the tritocerebral lobes are 

 reflected into the dorsal brain in many arthropods, 

 but not in Penaeus. In the latter the ganglia 

 remain in the primitive, divided condition, con- 

 nected by a transverse commissure, the tritocere- 

 bral (or stomodaeal) commissure (fig. 76), pass- 

 ing below the gut. The tritocerebral commissure 

 sends a small nerve, the stomodaeal nerve, pos- 

 teriorly from each side of the midline. The experi- 

 mental workers evidently prefer the somewhat 

 meaningless term, postcommissural nerve, for the 

 structure (Knowles 1953). Keim makes no men- 

 tion of the postcommissural, stomodaeal nerves in 

 Astacus. 



Associated with the tritocerebrum are the nerves 

 of the stomodaeal system. So far as this system 

 has been worked out in Penaeus, it appears to be 

 simple. A primary stomodaeal ganglion lying 

 on the anterior surface (primitive dorsal surface) 

 of the esophagus is joined to each tritocerebral 

 lobe by a nerve passing round the gut. From the 

 primary stomodaeal ganglion another nerve, the 

 recurrent nerve, passes dorsally along the dorsal 

 surface of the esophagus to the gastric mill where 

 it becomes subdivided. Keim finds a similar situa- 

 tion in Astacus. Henry (1948a) describes a sec- 

 ondary stomodaeal ganglion connected independ- 

 ently to the tritocerebrum, and from her work on 

 lower Crustacea we may conclude that the pres- 

 ence of two stomodaeal ganglia is the more typical 

 situation. 



Posterior to the tritocerebral ganglia the cir- 

 cumesophageal connectives passing around the gut 

 anastomose to form the ventral nerve cord. 



