52 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



of pleurosternal invaginations or apodemes in 

 decapods as the endophragmal system. In Asta- 

 cura and Brachyura these apodemes fuse inter- 

 nally to form complex endoskeletons consisting 

 of septate structures in the thoracic segments 

 above the ventral nerve cord. The sternal apo- 

 demes fuse in the midline to form the sternal 

 furca in the Insecta. In the gnathothorax of 

 Penaeux the laterotergal and sternal apodemes 

 are light and do not fuse, consequently no endo- 

 phragmal system is found, unless we consider the 

 transverse mandibular apodeme (see endosternite, 

 fig. 38) an endophragm of some sort. This 

 structure will be considered more fully in the 

 treatment of the mandibles. 



Although the ventral sternal elements of the 

 penaeid gnathothorax are not coalesced into a 

 rigid keel as in Astacura, the sternal plates do 

 broaden from the anterior to the posterior ends 

 of the gnathothorax, and abruptly so in the last 

 three thoracic segments. The sternal plates of the 

 last two thoracic segments are particularly modi- 

 fied in the female to receive the spermatophore 

 from the male (figs. 28, B ; 89). These structures 

 will be discussed in detail in the section on the. 

 reproductive organs, page 155. 



Gills 



The gills may be exposed by cutting away the 

 hranchiostegal region of the carapace along the 

 dorsal-most reaches of the inner lining of the 

 branchiostegite, where the lining joins the lat- 

 erotergal plates. The gills are thereby found to 

 occupy a chamber (fig. 31), open to the outside 

 ventrally and posteriorly by a narrow slot between 

 the leg bases and thoracic wall on the inside and 

 the extreme margin of the branchiostegite on the 

 outside. The chamber is closed dorsally by the 

 hranchiostegal fold. The chamber is rather shal- 

 low transversely in its broad, posterior region, 

 but becomes narrow anteriorly and is made much 

 deeper in the region of the second maxilla by the 

 lateral and horizontal reflection of the latero- 

 tergal, pleural plates. This narrow, deep, an- 

 terior chamber is thus a funnel, closed dorsally 

 by the pleural bridges, medially by the vertical 

 pleural wall, laterally by the branchiostegite, and 

 ventrally by the large, flat coxopodite exites (figs. 

 29, E; 42) of the first maxilliped. Inside the 

 funnel resides a pump, the scaphognathite of the 

 second maxilla (figs. 29, D; 31; 41). Details of 



its mechanical action will be considered when the 

 scaphognathite muscles are described, page 69. 



The tightly packed gills or branchiae rise 

 roughly dorsad from their points of origin on the 

 leg bases and pleura (fig. 31). Each gill consists 

 of an axial circulatory rachis from which the in- 

 dividual gill filaments branch. Details of gill 

 structure will be given below in the section on 

 respiration. Interspersed among the gills are six 

 flat, setose, bilobed structures, the mastigobran- 

 chiae or epipodites. The lateral margins of the 

 mastigohranchiae may be seen upon removal of 

 the branchiostegite (fig. 31), but they are best 

 seen if the gills are removed ( fig. 32) . If a shrimp 

 whose branchiostegite has been removed is cooled 

 so that the body processes are reduced but not 

 stopped, the long, fine setae of the mastigobran- 

 chiae may be seen to beat in phase with the move- 

 ments of the scaphognathite, or gill bailer, thus 

 suggesting that the epipodites play a part in the 

 water flow and cleaning of the gills. 



The older literature of Crustacea abounds in 

 so-called branchial formulae, the formal study of 

 gill origins. In the decapods the generalized situ- 

 ation is 4 gills for each side of the segment (fig- 

 32) ; 1 arises from the laterotergal. pleural plates 

 (pleurobranchia), 2 from an articular element 

 dorsal to the coxopodite (arthrobranchia), and 1 

 from the coxopodite (podobranchia) (Caiman 

 1909; Snodgrass 1952). The epipodite (mastigo- 

 branchia) arises from the coxopodite. As shown 

 by Caiman (1909), the use of the gill origins as 

 evolutional landmarks is limited by the practical 

 difficulty of distinguishing between the pleuro- 

 branchiae and arthrobranchiae in different 

 species, since ontogenetic changes of gill origin 

 are frequently seen. In fact, evidence exists sug 

 gesting that all the gills develop from the ap- 

 pendages, rather than from the limb bases or body 

 wall. Apparently in Penneus the podobranchiae 

 develop embryologically from the mastigohran- 

 chiae. 



As may be seen in figure 31, the branchial for- 

 mula for Penaeus setifems consists of 1 tiny ar- 

 throbranchia on the first maxilliped (fig. 42) ; 1 

 podobranchia, 1 mastigobranchia, and 2 arthro- 

 branchiae on the second maxilliped; 1 mastigo- 

 branchia, 2 arthrobranchs, and 1 pleurobranch on 

 the third maxilliped, and the first, second, and 

 third pereiopods; 1 arthrobranchia and 1 pleuro- 

 branchia on the fourth pereiopod, and 1 pleuro- 

 branch on the fifth walking leg. In a European 



