WHITE SHRIMP FROM THE GULF OF MEXICO 



"When compared externally with the crustaceans 

 mentioned (figs. 1, 2, 3), Penaeus setiferus is most 

 similar to Pandalus danae, a caridean prawn, and 

 less like Astacus astacus. The relationship with 

 the blue crab is obviously distant. The Penaeidae 

 are classed in the Sub Order Xatantia together 

 with the Caridae and the Stenopidae. All of 

 these are relatively strong swimmers with light 

 cuticles. Although Astacus displays many simi- 

 larities in form to Penaeus, the crawfish is a rep- 

 tant form with a heavy cuticle. With the lob- 

 ster, the crawfish has an extensively developed 

 chela on the first walking leg. whereas the white 

 shrimp bears a small chela similar in size to those 

 on the other chelate legs. . The antenna] scale and 

 the pleopods of the crawfish are much smaller 

 than those of the white shrimp. The former are 

 large in the white shrimp. In other general de- 

 tails, superficially, the white shrimp and the craw- 

 fish are substantially similar in structure. 



In general, the present anatomical study indi- 

 cates that the Penaeidae are relatively generalized 

 decapod crustaceans. Compared with the higher 

 decapods, the penaeids tend to have several parts 

 to accomplish a functional end that is carried out 

 by a single part in a higher representative. Ex- 

 pressing this in terms of phylogeny, the lower 

 decapod has lost fewer structures by the fusion of 

 parts than has the advanced form. Since the pres- 

 ent study of Penaeus is largely grounded upon 

 earlier work on higher decapods, the process of 

 homologizing the structures tends to be reversed 

 from the phyletic order. Homologies must there- 

 fore be drawn from the specific to the general. 

 Among the problems thus raised is the matter of 

 functional nomenclature, in particular, of the 

 muscles. 



The historical base for the naming of decapod 

 muscles is, for all practical purposes, the work 

 of Schmidt (1915) on Astacus. This investigator 

 employed a system of Latinized functional names, 

 handed down to him by earlier anatomists, for the 

 muscles he encountered in the European crawfish. 

 With minor exceptions Schmidt's nomenclature 

 accurately describes the actions of the muscles of 

 Astacus. However, the functional muscle names 

 of Schmidt do not describe the actions of the same 

 muscle having a different function in another 

 form. The investigator is therefore faced with 

 the decision either to transfer to a muscle in an- 

 other animal the functionally inaccurate name of 

 Schmidt, which will simplify comparisons, or to 



rename the muscle in each case in accordance with 

 its specific function, which will tend to compound 

 the existing confusion in morphological nomen- 

 clature. With no great pride I have chosen the 

 latter course in the present study. Until such 

 time as a system of nomenclature having univer- 

 sal validity can be devised, the dilemma is in- 

 escapable. 



Included in this paper is a section of biblio- 

 graphical references. These items are primarily 

 systematic, morphological, and experimental 

 papers on Crustacea which contain valuable ana- 

 tomical information used in the preparation of 

 the. present study, but not <uted specifically. Since 

 workers in many fields have to resort to anatomi- 

 cal studies in the course of their research, the ana- 

 tomical information is necessarily disguised un- 

 der titles which reflect the primary objects of 

 their research. The student of crustacean mor- 

 phology therefore, finds bibliographical compila- 

 tions of works from diverse sources very helpful. 

 The bibliography is in no sense complete. 



I. SKELETAL AND MUSCLE SYSTEMS 



The great mass of the shrimp body is comprised 

 of skeleton and, in particular, muscle: accord- 

 ingly, the bulk of the present anatomical study is 

 devoted to a consideration of these elements. The 

 description is presented in the order of the three 

 natural body regions of the animal, the simple 

 head, or protocephalon, the gnathothorax, and 

 the abdomen. The skeleton falls easily into these 

 divisions. The muscles, of course, do likewise, but 

 not so obviously, since many of them cross skeletal 

 subdivisions for mechanical reasons. In some 

 anatomical works, tliQ arthropod appendages are 

 treated separately, as if these organs were attached 

 to the animal in a kind of evolutional afterthought 

 in the arthropod line. The. appendicular muscles 

 would, indeed, so indicate, but the skeleton, the 

 nervous system, and the innervations of the 

 muscles tell us that the arthropod appendage is 

 an ancient structure. The appendages, then, will 

 be taken up with the tagmata to which they be- 

 long. 



A. Protocephalon 



The protocephalon is that morphologically 

 separable pregnathal group of segments so desig- 

 nated by Snodgrass ( 1951) . This simple head in- 

 cludes, in the order of their occurrence in the 

 adult, the eyes, antennules, antennae, and labrum. 



