WHITE SHRIMP FROM THE GULF OF MEXICO 



69 



3b. The Second Maxillae 



The second maxilla is one of the most extensive- 

 ly modified appendages of higher Crustacea. The 

 structure serves a double function. The medial 

 lobes participate in feeding while the large lateral 

 part pumps water over the gills. Despite its com- 

 plexity, the second maxilla is so remarkably uni- 

 form in structure and musculature throughout a 

 broad phyletic spectrum of higher Crustacea that, 

 as Caiman (1909) has shown, the appendage 

 is of limited value in the study of crustacean 

 evolution. 



SKELETAL ELEMENTS 



The second maxilla projects laterally and ven- 

 trally from its attachment point on the ventral 

 skeleton. The large foramen enters a deeply 

 sculptured coxopodite from which two small, 

 medial gnathal lobes arises (figs. 29, D; 41). 

 Distal to the coxopodite lies the complex basi- 

 podite bearing two larger, medial gnathal lobes 

 and an anterior endite (figs. 29, D; 41). Spines 

 on the gnathal lobes aid in holding food. Lateral 

 to the basipodite lies the flat, indented scaphog- 

 nathite, or gill pump (fig. 41). The folds and 

 grooves of the scaphognathite represent areas of 

 muscle attachment and of articulation. 



Many different interpretations of the compo- 

 nents of the second maxilla are encountered in the 

 literature. In Astacus, Schmidt describes the 

 gnathal lobes as partly coxal and partly basal, as 

 had been done in the present study of Penaeus. 

 However, the former considers the basipodite 

 endite the endopodite, and is followed by Berke- 

 ley in Pandalus. Caiman (1909) refers to the 

 endite as a palp. The position of Cochran (1935) 

 in her study of Callinectes is rather inconsistent. 

 When describing the second maxilla of the blue 

 crab, this worker calls all of the gnathal lobes 

 endites of the coxopodite, and terms the basip- 

 odite endite the endopodite. By contrast, in a 

 description of the mouthparts of a number of 

 crustaceans included as a subsection of the study 

 of the blue crab musculature, Cochran labels the 

 median gnathal lobes as basal and the anterior 

 lobe as an endite of the basipodite, as has been 

 done by the present writer in Penaeus, even 

 though the second maxillae of Callinectes and 

 Penaeus are very similar. To say the least, the 



composition of the second maxilla needs 

 clarification. 



MUSCLE ELEMENTS 



The principle function of the muscles of the sec- 

 ond maxilla is the operation of the scaphognath- 

 ite as a gill pump. As mentioned above, the gill 

 pump lies in a narrow channel through which the 

 water is moved that passes over the gills. If the 

 body processes of a shrimp are reduced by chill- 

 ing, and the branchiostegite is cut away, the gill 

 pump may be observed in slow action. Two func- 

 tionally interrelated but distinct oscillations of the 

 scaphognathite occur. The more obvious is that 

 taking place about the horizontal, lateral axis 

 through the gill pump, and by which the scaphog- 

 nathite is tipped back and forth, or rotated on 

 its axis. The less obvious oscillation is the dorso- 

 ventral movement of the lateral margin of the 

 scaphognathite about the long axis of the struc- 

 ture. The marginal undulation tends to ramify 

 the former oscillation with respect to water pump- 

 ing. 



Water is drawn into the pumping chamber by 

 a forward tipping of the anterior end of the gill 

 pump to the floor of the chamber. At the same 

 time the posterolateral margins of the pump are 

 raised to the top of the chamber. The postero- 

 lateral margins of the pump are now brought 

 ventrad at which time the anterior end rises, and 

 the whole organ rolls anteriorly along the floor 

 of the chamber, forcing water out of the cephalic 

 aperture of the pumping chamber. 



Conflicting opinions about the skeletal nomen- 

 clature of the second maxilla have given rise to 

 some confusion in the naming of the muscles. In 

 addition, small but important differences in the 

 number, arrangement, and in particular the func- 

 tions of the second maxillary muscles of Penaeus 

 are apparent when the second maxilla of the white 

 shrimp is compared to that of Astacus, Pandalus, 

 and Callinectes. The differences are of sufficient 

 magnitude to make difficult the homology of all 

 the muscles in Penaeus with those in the three 

 crustaceans mentioned without knowing the de- 

 tails of comparative innervations. Under pain of 

 causing further confusion of names in the litera- 

 ture, the present writer renames the muscles of 

 the second maxillae of Penaeus in accordance with 

 their functions. 



