FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



the Bears Bluff Laboratories, Wadmalaw Island, 

 S. C. ; and Di\ Milton Fingerman, Department of 

 Zoology, Tulane University, have all made useful 

 comments about various aspects of the work. I 

 am indebted to Prof. L. B. Holthuis, Rijksmuse- 

 um van Natuurlijke Historie, Leiden; Dr. Andre 

 Mayrat, Laboratoire de Zoologie de l'Ecole Nor- 

 male Superieure, Paris; and Jerome E. Stein, 

 Texas A. and M. College, Galveston, Tex., for 

 helpful criticism of certain parts of the study. 



Heartfelt thanks go to the project artist, Ray- 

 mond Bollinger of New Orleans, La., for the high 

 quality of the plates making up the anatomical 

 study of the white shrimp. Of the 136 figures in 

 this work, all but 10 were finished by Mr. Bol- 

 linger from tracings made by the investigator. 



METHODS 



The anatomical study oi'Penaeus sztiferus was 

 made for the most part on white shrimps pur- 

 chased alive from bait shrimp fishermen. The 

 animals were fixed in Zenker's fluid, dehydrated 

 to 70 percent ethyl alcohol and there stored. De- 

 spite difficulties in its use in the field, Zenker's 

 fluid was found to have several advantages over 

 formalin. Zenker's fluid softens or removes the 

 calcareous deposits and leaves the cuticle in a con- 

 dition similar to thick cellophane. This mix- 

 ture quickly penetrates to and fixes the internal 

 organs, and in so doing prevents internal 

 maceration caused by the post-morten enzymatic 

 activity of the hepatopancreas. Formalin-fixed 

 material is useless for the study of internal or- 

 gans. The fixative greatly hardens the cuticle 

 and the external muscles and fails to penetrate 

 to the internal organs. 



Dissections were performed under a stereo- 

 microscope. Dissecting needles which were 

 sharpened to fine points in mixtures of strong 

 nitric acid and ethyl alcohol were employed. Lo- 

 cations of muscles and attachments, and other 

 skeletal details were studied on specimens of 

 white shrimps cleared in strong alkali and stained 

 with Van Gieson's Triple Stain (Curtis' Modi- 

 fication). The outlines of whole structures were 

 used as templates within which muscles and other 

 organs were sketched in layers on tracing paper 

 as the dissections progressed. The tracings were 

 transferred to drawing papers on a light box. The 

 drawings were finished in ink and carbon pencil. 



PENAEUS AS COMPARATIVE MATERIAL 



For purposes of comparative morphology, the 

 Penaeidae enjoy a unique position. Evolution 

 has brought them down to us in an appareriwy 

 generalized decapod condition. Naturally, we 

 must view with caution any attempt to force a 

 given structure or organ into the generalized 

 category, for all extant animals must be highly 

 specialized in specific instances, if unspecialized 

 in others, for life today. Furthermore, the so- 

 called generalized structure may be the super- 

 ficies of a well-hidden specialization. Bearing in 

 mind, then, that there may be no such thing as a 

 generalized structure in a modern species, the 

 comparative morphologist can proceed to draw 

 homologies between structures which look alike on 

 phylogenetic and ontogenetic grounds. Informa- 

 tion shedding light on the evolution of the decapod 

 crustaceans will be advanced here wherever sup- 

 porting studies are available. 



Unfortunately, very few complete anatomical 

 works exist on decapod crustaceans, and none of 

 these complete works deal with members of the 

 Penaeidae. The present work will have reference 

 to the studies of many workers, but in particular 

 to the extensive studies of Berkeley (1928) on the 

 "coon stripe" shrimp, Pandalws danae Stimpson 

 1857, of Schmidt (1915) and Keim (1915) on 

 Antaeus astacus (Linn. 1758), and of Cochran 

 ( L935) on the blue crab, CalUnectex sapidus Rath- 

 bun, 1896. Other, frequently important, studies 

 will be alluded to in the applicable sections. The 

 greatest handicap to the establishment of a com- 

 parative morphology of the Decapoda is our igno- 

 rance of the details of the nervous system, the in- 

 nervations of muscles and other organs. Despite 

 widespread morphological change such as shifting 

 of muscle origins, coalescence of metameres, and 

 other distortions to the primitive body plan, struc- 

 tures tend to retain their ancient innervations. A 

 muscle may move from one segment to another in 

 the evolution of a group. The same nerve will 

 usually continue to innervate it. For the nervous 

 system is morphologically conservative and thus 

 the most informative element in the historical 

 study of metazoan structure. Until a substantial 

 groundwork of neurological facts exists on the 

 decapod crustaceans, homologies between many 

 of the less obvious structures will rest on weak 

 "rounds. 



