36 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



tennal segment, or basipodite. Upon contraction 

 the promoter muscles raise the antennal scale and 

 endopodite segments dorsally. The muscles 

 actually function as levators. Without infor- 

 mation about the innervation, no homolog of the 

 distal promotor muscle is here suggested. 



SECOND ANTENNAL SEGMENT REMOTOR 



MUSCLES 



Figs. 10, 24 



At least three large antennal muscles arise on 

 the dorsolateral carapace just anterior to the 

 hepatic spine. These muscles, here considered the 

 second antennal segment remoter muscles (figs. 

 19, 24), run anterolateral^ to extensive insertion 

 areas in the lateral and posterior regions of the 

 second antennal segment (basipodite). The 

 lateral- and posterior-most of these muscles might 

 be thought to insert on the ventral margin of the 

 first antennal segment, in which case the two 

 muscles would be first segment, or coxopodite, re- 

 motor muscles. Repeated dissections in the area 

 indicate otherwise, however, and for this reason 

 the muscles are assigned to the basipodite. The 

 posterior-most remotor displays a definite torsion 

 as it passes from its lateral point of origin to a 

 posterior and even slightly medial insertion area. 

 While the function of all three remotor muscles 

 is actually the depression of the large scale, that 

 of the posterior remotor muscle may also include 

 adduction of the scale, together with depression. 

 Since the antennal scale of Penaeus bears a 

 major portion of the weight of water striking the 

 anterior end of the animal, large remotor (de- 

 pressor) muscles are needed to maintain the scale 

 in position. 



The homologies of the second antenna segment 

 remotors is made confusing by the functional 

 muscle nomenclature. A comparison of the func- 

 tion of the antennae in the crawfish and Penaeus 

 shows wide differences. The crawfish antennal 

 segments are constructed to permit extension 

 movements of the comparatively short, stiff flagel- 

 lum. One kind of extreme of this modification 

 has been achieved in the antenna of Palinurus, in 

 which form movability is combined with great 

 size and power for the protection of the animal. 

 As has been suggested, the antennal movements 

 of Penaett* are comparatively limited by virtue of 

 segmental architecture, particularly in the seg- 

 ments of the protopodite. The heavy muscula- 

 ture of carapace origin is in reality associated 



with the simple movements of the scale; the mus- 

 cles of the distal antennal segments of the endo- 

 podite are comparatively small. 



As a consequence of the many antennal func- 

 tions in different crustaceans, homologous mus- 

 cles have different functions and nomenclature. 

 At least part of the second antennal segment re- 

 motor muscle mass in Penaeus is undoubtedly 

 homologous with the museums remotor II anten- 

 nae in Astacus. Pandalus, and Callinectes. Part 

 also may be homologous with the musculus de- 

 pressor c II antennae in Astacus and perhaps even 

 with a part of the large musculus depressor c II 

 antennae in Panda/us, although certainly the 

 major part of the antennal depressor c in Panda- 

 Jus is the protocephalon attractor muscle. The 

 other antennal depressor muscles, a, b. and d (d 

 is not found in Pandalus) in Astacus and Panda- 

 lus are not evident in Penaeus. 



SCALE ABDUCTOR MUSCLES 

 Figures 19, 20, 24 



Taking origin from large areas to the posterior, 

 ventral, and medial region of the second antennal 

 segment (basipodite), the proximal scale abductor 

 muscles (figs. 19, 20, 24) insert on the lateral mar- 

 gin of the antennal scale (exopodite) , lateral to the 

 dorsoventral scale condyles. In addition to the 

 huge ventral scale abductor, at least two and prob- 

 ably three small-scale abductors (figs. 20, 24) are 

 found in the second antennal segment of Penaeus. 

 A long, distal scale abductor muscle (fig. 24, B), 

 originating in the distal part of the scale, runs 

 proximally along the lateral margin of the scale 

 to insert on the lateral margin of the basipodite 

 foramen, lateral to the axis of the scale condyles. 

 When the scale abductor muscles contract, the 

 large scale is swung laterally some distance. The 

 functional reason for this movement is not clear. 

 Shrimps in an aquarium occasionally spread the 

 scales, at times in association with cleaning activi- 

 ties of the head appendnges and a sudden flushing 

 out of tlie gill chamber. 



The proximal scale abductor muscles of Penaeus 

 are probably homologous with the second antennal 

 exopodite abductor muscles a. b. and c in Astacus 

 and with the single exopodite abductor in Pan- 

 dalus. The scale is reduced in Callinectes. 

 Schmidt ( 1915) does not show a distal scale abduc- 

 tor muscle in Astacus such as exists in Penaeus, and 

 Berkeley (1928) makes no mention of the muscle 



