WHITE SHRIMP FROM THE GULF OF MEXICO 



47 



Although not shown in the illustrations of the 

 labrum, material of a glandular nature is found 

 in median ventral regions of the structure. Gland 

 cells in the crustacean labrum have been described 

 in the past. 



B. Gnathothorax 



The gnathothorax will be considered here as 

 those segments following the protocephalon in- 

 volved primarily with feeding and walking. The 

 latter are not truly separable as body tagmata in 

 the Crustacea, since after the mandibles and max- 

 illae, varying numbers of walking legs may be 

 adapted for feeding (Snodgrass 1951 ). Walking 

 legs adapted for feeding are referred to as niaxil- 

 lipeds. In the Crustacea Decapoda, the gnath- 

 othorax is comprised of the mandibles, paragna- 

 tha, •_' pairs of maxillae, 3 pairs of maxillipeds, 

 and 5 pairs of walking legs. In the present study, 

 the gnathal segments will include the maxillipeds, 

 and to the thorax will be assigned the live walk- 

 ing legs, without implying any morphological 

 rigidity to the division. Above and to the sides, 

 tli" gnathothorax is protected by the large dorsal 

 shield, or carapace. Nearly all trace of segmenta- 

 tion has disappeared from the dorsal regions of 

 the gnathothorax and the carapace. The few re- 

 maining sutures and markings of the carapace are 

 not well understood in the Crustacea: consequent- 

 ly systematic nomenclature which has grown 

 up around these devices is highly artificial. 



The gnathothorax is constructed in the form 

 of a rather special box arranged to provide both 

 rigidity and movability. The immobile carapace 

 is heavily sclerotized for protection of the internal 

 organs and support of muscle origins, and ex- 

 tends ventrad in a deep fold of the tergum to cover 

 completely the laterally placed gills (tigs. 30, 31). 

 The lateral carapace is called the branchiostegite, 

 since the structure forms a chamber for the gills. 

 The deep fold of the rigid carapace forming the 

 bianchiostegite permits movement between the 

 carapace and the architecture of the ventral 

 gnathothorax. The whole is reminiscent of a 

 modern sedan in which a rigid body above is at- 

 tached to a chassis able to respond to imperfec- 

 tions in the road surface. Upon the dorsal cara- 

 pace orginate numerous important muscles, in- 

 cluding some of those of the protocephalon ap- 

 pendages, the mandibles, maxillae, gastric mill, 

 and dorsal and ventral abdominal muscles. 



Compared to the ventral skeleton of Astacura 

 and Brachyura, that of /'< not us is very lightly 

 sclerotized (Snodgrass 1952; Huxley 1906). In 

 the crawfishes, the median sternal elements are 

 rigidly fused together, except for the sternum of 

 the last thoracic segment, and thus provide a rigid 

 keel from which the pleurosternal arms arise. 

 Pleural (laterotergal) and sternal apodemes arise 

 from the invaginations between the fused arms of 

 two adjacent segmental units. Similar apodemes 

 occur in the same locations in the ventral skeleton 

 of Penaeux. but the ventral sternal element of 

 each segmental unit is separated from its neigh- 

 bors by a transverse slit of thinly developed 

 cuticle (fig. 28). The slits permit movements be- 

 tween segments along the anteroposterior axis of 

 the thorax, even though the lateral pleurosternal 

 arms are fused. 



The ventral "skeleton of the gnathothorax in 

 Penaeus (figs. 28, 30) consists of a series of 

 sclerotized units which are slightly movable with 

 respect to one another, but not articulated (fig. 

 •28). Each unit bears the paired foramina and 

 muscle apodemes of the jointed appendages at- 

 tached thereto. The typical segmental unit con- 

 tains contributions from two sources: The tergum, 

 in the form of the dorsal tergum, and the vertical, 

 laterotergal pleural plates (Snodgrass 1952), 

 dorsal to the leg bases, and the sternum which 

 comprises the ventral region between the leg bases. 

 The dorsal condyles of the coxopodites are situ- 

 ated on the laterotergal plates while the ventral 

 coxopodite condyles are sternal. In the anterior 

 region of the gnathothorax of Penaeus, the 

 pleural plates lie horizontally to become the roof 

 of the gill chamber, similar to the arrangement in 

 Cambarus longulus Girard as shown by Snod- 

 grass (1952). 



The delicate nature of the ventral skeleton in 

 Penaeus is even more noticeable in lateral view 

 (fig. 28, ^1). From here the pleurosternal arms 

 may be seen to bifurcate in the pleural region 

 and those of each segmental unit tend to unite 

 dorsally in the form of a reverse-curve, or ogee, 

 arch of the architect. Again, compared to the 

 composite pleura of the crawfishes (Snodgrass 

 L952), those of Penaeus appear to have been re- 

 tained in a somewhat more generalized condition, 

 with 7 or 8 pleura clearly distinguishable from 

 one another (figs. 28, .10). 



Huxley (1906), Caiman (1909), and other 

 students of Crustacea often refer to the system 



