WHITE SHRIMP FROM THE GULF OF MEXICO 



105 



The posterior joints permit free extension and 

 flexion, but probably resist lateral motion. We 

 have, then, 4 kinds of intersegmental hinges func- 

 tioning in 2 different ways. No reason for this 

 multiplicity of structure in the hinges is readily 

 apparent. 



SKELETAL ELEMENTS 



The abdominal segment of arthropods is com- 

 prised of a ring of cuticle. This ring, however, 

 may be subdivided in parts. The abdominal 

 somite in the Crustacea has usually been said to 

 consist of an arched dorsal tergum and a flat, con- 

 cave, or convex ventral sternum connected to the 

 tergum by two lateral pleura, the latter often pro- 

 duced ventrad in a fold. The lateral pleuron has 

 been variously interpreted. Snodgrass (1935) 

 describes the pleural areas of arthropods as "typ- 

 ically membranous" to permit the movement of 

 appendages arising from the pleuron. Pleural 

 sclerites, when present, represent a contribution of 

 the proximal parts of the appendages, according 

 to this worker. He sets forth the fact also that the 

 appendages articulate with the ventral margins of 

 the pleural sclerites and with the lateral edges of 

 the sternal sclerites. That this typical situation 

 may not always obtain is shown by Hart (1952) 

 who found that the first pleopod in Cambarus 

 longulus longulus Girard, 1852, is connected to the 

 abdominal venter entirely by sternal components. 



Snodgrass ( 1935) apparently was of the opinion 

 that the arthropod pleuron represented a dis- 

 tinct region of the segment. In a study of the 

 Crustacea made at a later date, Snodgrass (1952) 

 describes in Anaspides tasmaniae Thomson, 1892, 

 a generalized malacostracan, a clearly demarked 

 pleural sclerite with which the limb basis articu- 

 lates. In the same study he refers to the pleural 

 sclerite as a "laterotergal pleural plate" and states 

 that the pleuron of the crustacean belongs to the 

 tergum. Support of the more recent opinion of 

 Snodgrass (1952) that the pleuron of crustaceans 

 is tergal in origin may be found by a consideration 

 of the abdominal segment of Penaeus setiferus. 



The sternum and tergum are distinct regions 

 in the white shrimp, but no line or suture can be 

 seen which distinguishes a pleural component of 

 the abdominal segment (fig. 59). From this we 

 may conclude that the pleura are not needed in a 

 morphological construct of the abdominal seg- 

 ment of the crustacean. That the term, "pleuron," 



for the laterotergal plates has become embedded 

 ineradicably in the literature of crustacean sys- 

 tematics is therefore most regrettable. 



Beyond what has been brought out about the 

 general structure and articulation of the abdom- 

 inal segments, little need be said further except 

 for a brief mention of the modified posterior end 

 of the sixth abdominal segment. Unlike the five 

 anterior abdominal segments from which light 

 appendages project, the sixth abdominal segment 

 must be strong enough to bear the tail fan. For 

 this reason the posterior end of the sixth segment 

 is heavily sclerotized. In addition, the contents 

 of the sixth segment are largely devoted to me- 

 diating the flexions of the tail fan which project 

 caudad from the segment rather than ventrad as 

 is the case with the preceding abdominal segments. 

 The result is the visible difference in shape of the 

 sixth segment compared to that of the typical ab- 

 dominal segment. More will be said about the 

 sixth abdominal segment when the tail fan is 

 considered. 



MUSCLE ELEMENTS 



The characteristic of the abdominal muscles, 

 apart from their great mass, is the unusual man- 

 ner in which they are laid out. These muscle 

 bodies are extraordinarily heavy and so inter- 

 twined with one another that their separation for 

 study is difficult. In accordance with the system 

 of Daniel (1931c), the abdominal muscles are 

 divided into the following four functional muscle 

 groups: (1) The superficial ventral muscles, (2) 

 the lateral muscles, (3) the dorsal muscles, and 

 (4) the main ventral muscles. They will be 

 treated in that order. 



SUPERFICIAL VENTRAL ABDOMINAL MUSCLES 



Figures 61, 63 



The superficial ventral abdominal muscles of 

 Penaeus are attached between yokes of thin cuticle 

 (fig. 63) lying transversely across the posterior 

 portion of each abdominal segment beneath the 

 ventral nerve cord. These yokes are associated 

 with the folds of articular cuticle on the ventral 

 surface, and they are so arranged that the super- 

 ficial ventral muscles run from the posterior re- 

 gion of one abdominal segment to the posterior 

 region of the next. The superficial ventral ab- 

 dominal muscles are of course a continuous sys- 



