72 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



ANTERODORSAL SCAPHOGNATHITE ROTATOR 

 MUSCLES OF SECOND MAXILLA 



Figure 41 



The anteroventral scaphognathite rotator mus- 

 cles have separate origins and may well be two 

 different entities. The posteriormost of the two 

 is a long muscle extending from the ventral 

 apodeme to an area well out in the anterior region 

 of the scaphognathite. The more anterior rotator 

 originates in the coxopodite and passes to a broad 

 area of insertion in the anterolateral region of the 

 gill pump. This muscle may be the counterpart 

 in Penaeus of the scaphognathite flexor muscle of 

 Astacus, Pandalus. and Callinectes. The poste- 

 riormost rotator of this pair in Penaeus appar- 

 ently is not homologous with any of the muscles in 

 the other crustaceans referred to. These muscles 

 bring the anterior tip of the scaphognathite 

 dorsad and promote the whole structure. 



ANTERODORSAL SCAPHOGNATHITE ROTATOR 

 MUSCLE OF SECOND MAXILLA 



Figure 41 



The anterodorsal scaphognathite rotator mus- 

 cle takes origin on the endosternal apodeme of the 

 second maxilla and runs anteroventrally to an 

 area of insertion mesad of the anteroventral rota- 

 tor muscles. The muscle reinforces the action of 

 the anteroventral rotators in raising the anterior 

 tip of the scaphognathite and promoting the 

 structure. As nearly as can be determined, the 

 anterodorsal rotator in Penaeus is the musculus 

 respiratorius a II maxillae in Astacus, Pandalus, 

 and Callinectes. 



4. MAXILLIPEDS 

 4a. First Maxilliped 



Like the second maxilla, the first maxilliped is 

 a highly modified appendage having a dual func- 

 tion. Its strong gnathal lobes and sensory flagel- 

 lum participate in feeding, while its flat, lateral 

 lobes and small arthrobranchia play a part in 

 breathing. Superficially, the first maxilliped ap- 

 pears to be as widely modified from the plan of 

 the typical appendage as is the second maxilla, 

 but the muscles indicate otherwise. And whereas 

 the second maxilla is a relatively stable phylo- 

 genetic entity in the Crustacea, the form of the 

 first maxilliped is variable. 



SKELETAL ELEMENTS 



The skeleton of the first maxilliped is comprised 

 of a series of lightly sclerotized lobes or plates. 

 The appendage articulates with the ventral skele- 

 ton by a transversely elongate foramen. The 

 foramen enters the fused coxopodite and basipo- 

 dite (protopodite, fig. 42). Projecting anteroven- 

 trally from the protopodite is a set of three 

 gnathal lobes, the large, thick distal lobe being 

 the endite of the coxopodite (fig. 42). Together, 

 the endites of the maxillipeds are cupped against 

 the anterior mouthparts. Heavy spines directed 

 mesad from the edge of the endites help to hold 

 food. Laterad of the coxopodite endite and ex- 

 tending anteriorly is a slender lobe bearing on its 

 medial surface the rudiment of the endopodite 

 and the jointed flagellum of the exopodite. The 

 exites of the coxopodite, lying laterally, are two 

 large, flat sheets that close the gill pump cham- 

 ber on the ventral surface. Posteriorly, a small, 

 flattened gill may be seen. This gill is said to be 

 an arthobranchia, but on embryological grounds 

 it might be as easily a podobranch. 



The components of the first maxilliped are in- 

 terpreted in different ways by different authors. 

 The structure called the protopodite in Penaeus 

 is termed the coxopodite in Astacus by Schmidt. 

 Also, Schmidt considers the coxopodite endite the 

 basipodite, and refers to the coxopodite exites as 

 epipodites. If, as has been discussed earlier, the 

 coxopodite exites develop from podobranch pri- 

 mordia, then no objection to the term ''epipodite v 

 can be offered. In passing, we may note that the 

 first maxilliped of Penaeus bears two coxopodite 

 endites compared to one in the first maxilliped of 

 Pandalus, Astacus, and Callinectes. 



MUSCLE ELEMENTS 



Compared to the musculature of the second max- 

 illa, that of the first maxilliped is very light. The 

 muscles function almost entirely in feeding, since 

 the part played by the exites in breathing is most- 

 ly passive. Wider variation in the functions of 

 the muscles of the first maxilliped in different 

 crustaceans makes necessary a variety of muscle 

 names, but most of them can be homologized. The 

 first maxilliped of Penaeus has 12 muscles, against 

 9 for Astacus, 13 for Pandalus, and 11 in Cal- 

 linectes. V 



