WHITE SHRIMP FROM THE GULF OF MEXICO 



139 



Ventral Nerve Cord 

 Figures 36, 61, 63, 76 



The ventral nerve cord of Penaeus is a longi- 

 tudinal series of ganglia interconnected by fused 

 pairs of intersegmental nerve tracts. A certain 

 amount of longitudinal coalescence lias taken place 

 in the ventral cord, notably in the anterior, tho- 

 racic regions and less so in the abdomen. The 

 ganglia of the gnathal appendages are conspicu- 

 ously fused. 



PARAGNATHAL NERVE 



FIGURE 76 



The element termed the nerve to paragnath 

 arises from the ventral cord anterodorsad of the 

 mandibular nerve. In Penaew the paragnathal 

 nerve is in no way posterior to the mandibular 

 nerve as shown by Henry (1948a) for various 

 crustaceans. However, whether the nerve is his- 

 tologically anterior to the nerve to the mandible, 

 as Chaudonneret (1956) holds, is not known. If 

 Chaudonneret is correct in his claim that the 

 paragnatha are anterior to the mandibles, then 

 we are faced with the necessity of explaining the 

 segmental interrelationships suggested by this 

 view. Chaudonneret escapes from the dilemma 

 by homologizing the paragnatha with the insectan 

 superlinguae, a theory that has been advanced in 

 the past (Snodgrass 1935). At the same time, 

 Chaudonneret stoutly defends the morphological 

 independence of the paragnathal nerve from that 

 of the mandible. He thus arrives at a paradox, 

 since the superlinguae arise from the hypo- 

 pharynx and appear to belong to the mandibular 

 segment (Snodgrass 1935) . If, on the other hand, 

 the superlinguae are not mandibular, and their 

 homology with the paragnatha holds good, we 

 must assign the paragnatha to some other seg- 

 ment, perhaps pre-mandibular and almost cer- 

 tainly postoral. 



MANDIBULAR NERVE 



Figures 36, 76 



The large nerve to mandible is given off the 

 ventrolateral surface of the anterior ventral nerve 

 ganglion and runs into the heavy muscles of the 

 mandibular segment. A branch of the nerve 

 enters the mandibular palp. 



MAXILLARY NERVES 



Figures 36. 76 



The nerves to the first and second maxillae de- 

 part from the ventral nerve cord and pass into 

 the maxillae. The nerves to the first maxilla arc 

 much smaller than the one to the more active sec- 

 ond maxilla. These nerves are very likely mixed 

 since they separate into branches which run to 

 muscles and to sensory end cells in the appendage 

 extremities. 



MAXILLIl'EDAL NERVES 



Figures 36, 43 to 48, 76 



The nerves to the first and second maxillipeds 

 leave the anterior ventral nerve ganglion ventro- 

 laterally from its caudal portion and run into the 

 maxillipeds. The nerve to the third maxilliped 

 arises from its own ganglion. 



NERVES TO I'EREIOPODS 



Figures 36, 40, 51 to 53, 56 to 58 



Beginning with the ganglion of the third max- 

 illipeds, the thoracic ganglia are indicated for the 

 most part by independent swellings in each seg- 

 ment. From the ventral surface of each ganglion 

 a large nerve departs into each appendage. Be- 

 tween the ganglion of the third thoracic segment 

 and. the anterior portion of the coalesced fourth 

 and fifth ganglia, the two longitudinal nerve tracts 

 have been retained in the primitive divided condi- 

 tion to permit the sternal artery to pass through 

 them. The thick ventral nerve cord of the thorax 

 narrows posterior to the fused ganglia of the 

 fourth and fifth thoracic segments and runs into 

 the abdomen. 



ABDOMINAL NERVES 



Figures 61. 63, 71. 72, 76 



The abdominal ganglia of the ventral nerve cord 

 are all substantially similar, except for the termi- 

 nal ganglion of the sixth abdominal segment. Two 

 nerves arise from each of the first five abdominal 

 ganglia. The anterior one enters the pleopod on 

 that side and the posterior nerve sweeps laterally 

 and dorsally to branch among the abdominal mus- 

 cles of the segment. 



The sixth abdominal ganglion lies in the pos- 

 teroventral portion of the sixth segment (fig. 72, 

 B). Four pairs of major nerves arise from it. 



