ABSTRACT 



In this illustrated morphology of the commercially important white shrimp of the 

 Gulf of Mexico the muscle, nervous, circulatory, excretory, reproductive, and respiratory 

 systems are compared with those of the blue crab, Callinectes, a European crawfish 

 Astacug, the "coon-stripe" shrimp, Pandalus, and other decapod crustaceans. The major 

 portion of the comparative work deals with the muscles, since the muscle systems of 

 a few Decapoda have been reported in much greater detail than other systems. 



The comparative studies of muscles and nerves indicates that the Penaeidae represent 

 a generalized anatomical condition in the Crustacea Decapoda, thus verifying the sys- 

 tematic research in this area. Evidently the Penaeidae are relatively close to the decapod 

 stem in the Malacostraca. The generalized condition of Penaeidae is shown again and 

 again by the repetition of functional muscle units. The same units have become simplified 

 in the higher decapods, having been lost, presumably, by the fusion of separate parts. 

 Adhering to the morphological principle that the nerves tend to retain their ancient 

 innervations despite coalescence of parts, shifting of muscle origins, etc., and can there- 

 fore be considered as morphologically conservative, the nerves to the repeating muscle 

 units of Poweiis are found to have kept their innervations to the same muscles, now 

 fused, in higher Decapoda. The comparative morphology of decapod nerves and circu- 

 latory elements is treated only to the extent that research on these systems in other 

 decapods has been published. 



Several structures are found in Penaeits setiferus which have not been reported 

 previously in the literature. A fibrous circulatory element, the capillary arbor, pene- 

 trates the distal optic ganglia. One or more hard concretions embedded in the substance 

 of the antennal portion of the excretory gland are described. Although two pairs of 

 muscles are associated with the labrum of Insecta, muscles have not been described in 

 the labrum of Crustacea. The labrum of Pctiaeus has at least 12 pairs of muscles. A 

 structure, the hindgut gland, is found in the anterior part of the sixth abdominal segment 

 lying dorsal to the rectum. Its function is unknown. Some of the blood vessels of the 

 heavily vascularized branchiostegal region of the carapace run parallel to the margin 

 of the carapace, suggesting "growth rings" by their appearance. 



