WHITE SHRIMP FROM THE GULF OF MEXICO 



79 



to a large sternal apodeme overhanging the an- 

 teromedial part of the foramen. The muscle 

 runs beneath the coxopodite promoter and inserts 

 together with the lateral basipodite levator on the 

 anterior margin of the basipodite foramen, 

 slightly laterad of the dorsal coxobasipodite 

 condyle. The lateral levator muscle originates on 

 the posterior wall of the coxopodite and runs 

 ventrad to join the medial levator. The muscles 

 raise the distal articles and also abduct them. The 

 muscles are homologous with the levator muscles 

 a and b of Pandalus and Astacus, and with the 

 single levator in CalUnectes. 



BASIPODITE DEPRESSOR MUSCLES OF SECOND 

 MAXILLIPED 



Figures 44, 45 



The second maxilliped of Penaeus contains four 

 basipodite depressor muscles. The largest is the 

 medial depressor, a short, strong, semicylindrical 

 muscle which, in a manner of speaking, lines the 

 medial curvature of the coxopodite. This muscle 

 originates from the medial margin of the dorsal 

 coxopodite foramen and inserts with the other de- 

 pressors on the ventromedial margin of the coxo- 

 podite, mesad of the ventral condyle between the 

 coxopodite and basipodite. Two other basipodite 

 depressors arise, the larger from the posterior 

 margin of the coxopodite foramen, the smaller 

 from the posterior wall of the coxopodite. The 

 fourth depressor muscle originates on the ventro- 

 lateral wall of the coxopodite, beneath the coxo- 

 podite remoter, and runs across the coxopodite to 

 join the other basipodite depressors. The con- 

 tractions of the depressor muscles turn the basipo- 

 dite and thus the distal maxillipedal elements 

 ventrad. In addition, as a consequence of their 

 insertion mesad of the condylic axis, the depressor 

 muscles turn the basipodite and the distal elements 

 towards the midline. 



In fact, from the arrangement of the basipodite 

 levators and depressors with respect to the basipo- 

 dite condyles, the true function of the muscles 

 may be as abductors and adductors, rather than 

 as levators and depressors. The latter names, of 

 course, derive from Astacus in which the coxopo- 

 dite and basipodite of the second maxilliped are 

 fused. On functional grounds, then, Schmidt's 

 names are at least partially incorrect when ap- 

 plied to Penaeus. The basipodite depressor mus- 

 cles of Penaeus are nonetheless homologous with 



the depressor muscles a and b in Pandalus. Asta- 

 cus, and I 'allinectes. 



EXOPODITE ABDUCTOR MUSCLES OF 

 SECOND MAXILLIPED 



Figure 45 



Two exopodite abductor muscles exist in the 

 second maxilliped of Penaeus. The medial ab- 

 ductor is extrinsic, originating broadly on the me- 

 dial wall of the basipodite and inserting on the 

 posterior edge of the exopodite base. The extrin- 

 sic abductor turns the exopodite laterad. Intrinsic 

 to the exopodite is a long abductor muscle which 

 originates by a fine tendon on the base of the 

 exopodite and extends distally for some distance 

 in the exopodite flagellum. Its contractions bend 

 the flagellum laterad. The intrinsic exopodite 

 abductor muscle in Penaeus is homologus with 

 the exopodite abductor muscle of Astacus and 

 CalUnectes, and the extrinsic abductor is proba- 

 bly the same muscle as the flagellum abductor in 

 Astacus. 



EXOPODITE ADDUCTOR MUSCLES OF SECOND 

 MAXILLIPED 



Figure 45 



The second maxilliped of Penaeus has at least 

 three exopodite adductor muscles. The extrinsic 

 exopodite adductor originates in a broad fan 

 across the dorsomedial wall of the basipodite 

 and inserts on the anterior edge of the exopodite 

 base. When it contracts, the exopodite is turned 

 towards the midline. Within the exopodite are 

 two exopodite adductors which function to 

 straighten out the exopodite flagellum, in oppo- 

 sition to the action of the intrinsic exopodite ab- 

 ductor muscle. No homolog of the extrinsic exop- 

 odite adductor muscle is evident in Astacus or 

 CalUnectes. Berkeley (1928) illustrates an ex- 

 trinsic adductor of the exopodite, but does not 

 discuss the muscle. The intrinsic exopodite ad- 

 ductors of Penaeus apparently have no homologs 

 in the crustaceans referred to here. 



MEROPODITE PRODUCTOR MUSCLE OF SECOND 

 MAXILLIPED 



Figure 45 



The types and arrangement of the second max- 

 illiped endopodite muscles are remarkably uni- 

 form in many higher Crustacea, in accordance 



