134 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



time, and for this reason, may be used by the mor- 

 phologist to bring order out of apparent phylo- 

 genetic chaos in the other organic systems. Gaps 

 exist in our knowledge of decapod neuroanatomy, 

 and frequent reference lias been made to the lack 

 of detailed information about innervations of mus- 

 cles and other structures in morphological work. 



The basis for an understanding of the history 

 of muscles and other structures is the exact and 

 full understanding of the details of the nervous 

 system. The literature of arthropod morphology 

 does not provide the facts, except in rare instances, 

 nor are all the anatomical facts about the nerves 

 of Penaeus made available in the present work. 

 The reason for this is the high technical skill and 

 great periods of time required to work out in de- 

 tail the gross anatomy and histology of a nervous 

 system. The 2 years devoted to the present re- 

 search on white shrimp have not been sufficient 

 for this purpose. 



Despite present shortcomings, a number of im- 

 portant details of the central nervous system of 

 Penaeus have been worked out. Typical of An- 

 nulata, the shrimp nervous system is comprised 

 of a dorsal brain connected to the ganglionated 

 ventral longitudinal nerve cord below the gut by 

 two large tracts. The gut passes between these 

 tracts. In general, the brain or supraesophageal 

 ganglion, receives nerves from the special sense 

 organs of the head and supplies nerves to the mus- 

 cles operating them. The first ganglion of the 

 ventral nerve cord, usually called the sub- 

 esophageal ganglion, together with the follow- 

 ing metameric ventral ganglia, receive impulses 

 from sensory end organs of the body and append- 

 ages and send motor impulses to the muscles mov- 

 ing these structures. 



The dorsal brain of annulates is variously com- 

 posed. The arthropod brain is usually said to con- 

 sist of an anterior protocerebrum containing the 

 nerve centers of the eyes and other preantennal ap- 

 pendages supposed to have existed in primitive 

 forms. The protocerebrum is joined to a second 

 brain part, the deutocerebrum, an area associated 

 with the antennules, or first antennae. In all in- 

 sects and most Crustacea, a third brain region, the 

 tritocerebrum, is added to the other parts. The 

 tritocerebrum has traditionally been said to be the 

 nerve center for the antennae (second antennae), 

 although Ferris (1953) presents evidence oppos- 

 ing this view. Classically, the tritocerebrum has 

 been considered the first ganglion of the ventral 



nerve cord due to the presence of a large postoral 

 commissure between the lobes of the tritocerebrum. 

 The tritocerebral lobes have thus moved around 

 the mouth to join the dorsal brain in many arthro- 

 pods. 



In some crustaceans, however, including Pe- 

 naeus setiferus, the tritocerebral lobes have not be- 

 come part of the dorsal brain and instead remain 

 ventrally located. Although clearly tritocerebral, 

 the ganglia do not send nerves to the second anten- 

 nae. 



The ventral nerve cord is the fusion product 

 of a "ladder" nervous system, wherein the paired 

 ganglia of each segment have come together at the 

 midline. Longitudinal segmental coalescence has 

 frequently been followed by ganglionic coalescence 

 in the ventral nerve cord with attendant obscuring 

 of primitive metamerism. 



In the following treatment, the nerves of the 

 dorsal brain and tritocerebrum will be considered 

 in the first section and those of the ventral nerve 

 cord in the second. 



A. Nerves of Supraesophageal Ganglion 

 and Tritocerebrum 



The supraesophageal ganglion, or dorsal brain 

 (figs. 75, 76), lies within the head lobe in the dor- 

 sal part of the protocephalon. The head lobe is 

 protected dorsally by the broadening base of the 

 rostrum. The dorsal brain is made up of nerve 

 cell bodies and tracts associated with the nerves 

 running out of it. 



TEGUMENTAL NERVES 



Figures 75, 76 



The tegumental nerves arise from slightly dif- 

 ferent points on the anterior face of the supra- 

 esophageal ganglion and run. directly rostrad to 

 the epidermis of the head lobe. Keim (1915) does 

 not show similar structures in Astacus. 



OPTIC TRACT 



Figures 6 to 10, 75, 76 



The optic tract, a part of the brain, rises from 

 the anterolateral region of the supraesophageal 

 ganglion, runs distally in the eyestalk, increasing 

 in diameter, and enters the calathus. Within the 

 calathus, the optic tract enlarges to incorporate 

 the various distal optic ganglia and makes contact 

 with the nerves from the ommatidia (figs. 9, 10). 



