28 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



ADDUCTOR MUSCLE OF LATERAL FLAGELLUM 



Figure 15 



Arising from the anteromesial margin of the 

 first antennular segment, just dorsal to the origin 

 of the second basal segment adductor muscle, is 

 the long, slender adductor muscle of the lateral 

 flagellum. The muscle courses anteriorly through 

 both the second and third antennular segments to 

 insert on an apodeme arising from the mesial edge 

 of the lateral flagellum base. Upon contraction 

 of the lateral flagellum adductor the lateral flagel- 

 lum is turned on its points of articulation toward 

 the mid-sagittal plane. The muscle has a prob- 

 able homolog in the musculus adductor I an- 

 tennae of Pamdalus, although in the latter the 

 muscle inserts in the ventral part of the third 

 antennular segment rather than on a flagellum. 

 A lateral flagellum adductor muscle does not oc- 

 cur in the antennule of Astacus or Callinectes. 

 Cochran (1935) describes a musculus adductor 2 

 I antennae for Callinectes that functions some- 

 what like the musculus adductor I antennae found 

 by Berkeley (1928) in Pandalus, but which is not 

 homologous with the latter muscle or with the 

 lateral flagellum adductor muscle in Penaeus. A 

 study of the nerves might show that the musculus 

 adductor, I antennae in Callinectes is part of the 

 musculus reductor 2 I antennae (second segment 

 adductor muscle of Penaeus) . 



ADDUCTOR MUSCLE OF THIRD BASAL SEGMENT 



Figure 14 



The relatively large adductor muscle of the 

 3d basal segment originates in the posteromesial 

 corner of the second antennular segment and runs 

 directly anterior to insert on an apodeme in the 

 posteromesial corner of the first antennular seg- 

 ment. Contractions of this muscle turn the third 

 antennular segment and the flagella mesad, rein- 

 forcing the action of the proximal antennular ad- 

 ductors. The third basal segment adductor of 

 Penaeus has homologs in the musculus reduc- 

 tor 3 I antennae of Pandalus, Astacus, and Cal- 

 linectes. Cochran's (1935) use of the name "re- 

 ductor" for the muscle in Callinectes is unfor- 

 tunate. The reductor muscle of the third anten- 

 nular segment in the blue crab appears to function 

 virtually opposite to the muscle of the same name 

 in Pandalms and Astacus, a situation that under- 

 lines the impropriety of transferring functional 

 muscle names from one animal to another. 



ABDUCTOR MUSCLES OF THIRD BASAL SEGMENT 

 Figures 14, 15 



The second antennular segment of Penaeus con- 

 tains three abductor muscles of the 3d basal seg- 

 ment. The lateral abductor muscle originates in 

 the posterolateral corner of the second antennular 

 segment and runs directly anterior to its point 

 of insertion on a small apodeme of the postero- 

 lateral corner of the third antennular segment. 

 Just mesad of the lateral, or first, abductor, a 

 second abductor muscle originates broadly on the 

 ventral surface of the posterior region of the 

 second segment. This second abductor muscle 

 courses anteriorly, parallel to the lateral-most 

 abductor muscle, and inserts on a short apodeme 

 slightly mesad of the insertion of the lateral-most 

 muscle (figs. 14, 15). The mesial third basal seg- 

 ment abductor muscle originates in the postero- 

 mesial corner of the second antennular segment 

 dorsal to the origin of the third basal segment 

 adductor muscle. The mesial abductor runs di- 

 agonally to the anterior to insert on the apodeme 

 of one of the lateral abductor muscles (fig. 14). 

 Contractions of the third segment abductor mus- 

 cles turn the third segment and the flagella away 

 from the mid-sagittal plane. Their action rein- 

 forces that of the first and second segment abduc- 

 tor muscles. 



The lateral-most abductor muscle of the third 

 antennular segment of Penaeus is homologous 

 with the musculus productor 3 I antennae of Pan- 

 dalus, Astacus, and jiossibly Callinectes. In Pan- 

 dalus and Astacus the musculus productor 3 

 swings the third segment dorsally in the sagittal 

 plane, while in Callinect-es the muscle named by 

 Cochran (1935) a "productor" appears to flex the 

 third segment. The second lateral third-segment 

 abductor muscle in Pena us has homologs in the 

 musculus abductor 3 I antennae of Pandalus and 

 Astacus. The muscle has been lost in Callinectes. 

 The mesial third-segment abductor muscle of Pe- 

 naeus does not appear in the antennule of any 

 of the crustaceans to which reference has been 

 made here. 



ABDUCTOR MUSCLE OF LATERAL FLAGELLUM 



Figures 14, 15 



The abductor muscle of the lateral flagellum 

 has its origins along most of the mesial side of 

 the third antennular segment. The muscle nar- 



