92 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



The lateralmost levator (fig. 50, .4) is a short, 

 strong muscle taking origin on the lateral edge 

 of the coxopodite and inserting on an anterolateral 

 apodeme of the basipodite. Mesad of this levator 

 lies a long, curving basipodite levator (fig. 50, A, 

 B) which originates on the pleural plate and joins 

 the former levator on the basipodite levator apod- 

 eme. The third basipodite levator is a short, 

 rounded muscle situated mesad of the previously 

 mentioned levators (fig. 50, A ) . The third levator 

 originates in the coxopodite and inserts on the 

 anterior rim of the basipodite. The medial, fourth 

 levator, also originating on the coxopodite, inserts 

 on the anteromedial edge of the basipodite (fig. 51, 

 .4 ) . The basipodite levator muscles turn the basip- 

 odite and the distal leg article dorsad. 



While the three walking leg levator muscles 

 found in Astacus and Callinectes are undoubtedly 

 homologous with some or all of the basipodite 

 levators in Penaeus, the details of their relation- 

 ships are uncertain. 



BASIPODITE DEPRESSOR MUSCLES 

 OP FIRST PEREIOPOD 



Figures 49, 50, 51 



The first walking leg of Penaeus has four basip- 

 odite depressor muscles. Two of the depressors 

 originate in the coxopodite and two on the pleural 

 wall. The lateralmost member of the group 

 originates on the lateral side of the coxopodite 

 (fig. 51, A) and passes ventromedially to insert 

 on the common basipodite depressor apodeme. 

 The. longer, second depressor originates dorsad, 

 on the pleural wall, and runs ventrad to the com- 

 mon apodeme. Mesad of the second basipodite 

 depressor lies a short third depressor of the basip- 

 odite which is attached between the dorsomedial 

 rim of the coxopodite and the basipodite apodeme. 

 The fourth basipodite depressor muscle lines the 

 medial side of the coxopodite. Unlike the other 

 depressors, the fourth basipodite depressor inserts 

 for some length along the posteromedial margin 

 of the basipodite. 



The basipodite depressor muscles turn the basip- 

 odite ventrad, thus accomplishing body support 

 on the limbs. Considerable variation in basip- 

 odite musculature exists. The depressor muscle 

 a of Astacus is very likely the homolog of the 

 long, second basipodite depressor of Penaeus, 

 whereas the depressor muscle b of Astacus is 

 probably homologous to the lateral, first basip- 



odite depressor muscle in Penaeus. The rela- 

 tionships in Penaeus to the seven depressor mus- 

 cles of the blue crab are difficult to ascertain. The 

 depressor muscle b in the third pereiopod of Pan- 

 dalus looks much like the medial, fourth basip- 

 odite depressor muscle in the first pereiopod of 

 Penaeus. 



EXOPODITE MUSCLE OP FIRST PEREIOPOD 



Figure 50 



The little, spindle-shaped exopodite muscle 

 originates on the wall of the basipodite, mesad, 

 and passes across the basipodite to a point of in- 

 sertion at the base of the exopodite. Presumably 

 its contractions move the exopodite, which, while 

 very reduced, is connected to the basipodite by an 

 articular joint. Nothing similar to the exopodite 

 muscle is described in the first pereiopod of the 

 other crustaceans referred to. Berkeley describes 

 an attractor of the mastigobranch in the pereio- 

 pods of Pandalus, but illustrates the muscle as a 

 coxopodite component. 



ISCHIOPODITE REDUCTOR MUSCLE OF FIRST 

 PEREIOPOD 



Figure 49 



The ischiopodite reductor muscle originates 

 over an extensive area on the dorsomedial surface 

 of the basipodite. The muscle inserts on a prox- 

 imal ischiopodite apodeme located on the ventro- 

 medial rim of the ischiopodite. The reductor 

 pulls the ischiopodite and with it the distal arti- 

 cles ventrad. The ischiopodite reductor muscle of 

 Astacus is the same as that of Penaeus. The 

 muscle is missing in the blue crab, in which the 

 basipodite and ischiopodite are fused. 



MEROPODITE FLEXOR MUSCLE OF FIRST 

 PEREIOPOD 



Figure 49 



The meropodite flexor muscle is a two-part 

 structure whose short fibers insert at an angle on 

 an elongate apodeme projecting from the ventro- 

 medial edge of the meropodite, proximally. The 

 apodeme divides the muscle approximately in 

 half on its long axis. The muscle fibers originate 

 about a wide area of the ventral and medial sur- 

 face of the ischiopodite. Their contractions turn 

 the meropodite mesad and to a lesser extent ven- 

 trally. The meropodite flexor muscle apparently 

 is subdivided into two parts in Astacus, in which 



