WHITE SHRIMP FROM THE GULF OF MEXICO 



33 



The fifth antenna] segment (figs. 18, 23) lies on 

 the axial line distal to the fourth antenna] segment 

 and is connected to the pointed fourth segment by 

 a broad V-shaped surface in the proximal end of 

 the fifth segment. Two condyles in the vertical 

 plane located at the apex of the V-shaped surface 

 permit limited horizontal movements of the fifth 

 segment on the fourth antennal segment. The rel- 

 atively large fifth antennal segment bears the base 

 of the long flagelluni on its distal end. Strong 

 dorsoventral condyles allow the flagelluni to turn 

 through an arc of more than 00° and the size of 

 the fifth segment is probably an evolutional re- 

 sponse to the long muscles needed to operate the 

 flagelluni. In living shrimps, the distal part of 

 the flagellum is carried at right angles to the fifth 

 antennal segment, the rest of the long flexible 

 flagellum floating behind the animals. The anten- 

 nal flagellum (figs. 23, 25) owes its flexibility to 

 its annular construction. Each small annulation 

 is capable of a little movement with respect to its 

 neighbors. 



An enlargement of the flagellar rings (fig. 25) 

 shows probable sensory structures. On the dorsal 

 surface of each ring (fig. 25, .4) may be seen a 

 pair of dorsal setae. On the ventral surface (fig. 

 25, B) a pair of plumose ventral setae project an- 

 teriorly from the distal portion of each annula- 

 tion. Between the bases of the ventral setae is a 

 ventral pit. The interior of the flagelluni consists 

 of blood vessels and nerves. 



MUSCLE ELEMENTS 



The skeletal parts of the antenna of Penaeus 

 setiferus are operated by 12 types of muscles, in- 

 cluding at least 26 individual muscles. Berkeley 

 (1928) describes 15 types of muscles in Pandalus. 

 Schmidt (1915) lists 18 muscle types in the Euro- 

 pean crawfish, the appendage containing 21 sep- 

 arate muscles to carry out its complex movements. 

 The reduced antenna of Callinectes has only 8 mus- 

 cles (Cochran 1935). The evolutional trend of re- 

 duction of the number of muscles and muscle types 

 has apparently been reversed in the case of the 

 crawfish antenna, in which form much more com- 

 plicated antennal activities are displayed by the 

 living animals than in Penaeus or Pandalus. 



FIRST ANTENNAL SEGMENT ADDUCTOR MUSCLE 



Figs. 21, 24 



Attaching on the median rim of the coxopodite 

 foramen, the first antennal segment adductor mus- 



cle inserts in the first antennal segment near the 

 external opening of the excretory apparatus (fig. 

 21 ) . Contractions of this short, powerful muscle 

 turn the coxopodite and hence the antenna mesad. 

 Homologs of this muscle in Astacus and Pan- 

 dalus are difficult to determine without full infor- 

 mation on the nerves. Allowing for functional 

 differences in the antennae of the various crusta- 

 ceans here considered, the most likely homolog 

 of the first segment adductors in Astacus is the 

 museums depressor a II antennae. Berkeley 

 (1928) designates two medial antennal base 

 muscles as the musculus depressor a and b II 

 antennae, after Schmidt (1915). From her illus- 

 trations of the antenna in Pandalus, the antennal 

 depressor muscle a appears to be the same muscle 

 as the first antennal segment adductor in Penaeus. 

 The muscles in the latter two forms are similar, 

 strongly suggesting an homology. The antennal 

 depressor muscle a in Astacus is much less sug- 

 gestive of phylogenetic similarity. 



SECOND ANTENNAL SEGMENT PROMOTOR 

 MUSCLES 



Figs. 19. 22 



Far and away the heaviest musculature of the 

 anterodorsal region of the white shrimp is that 

 concerned with the antennae. The dorsal-most of 

 these is a large, flat muscle originating in connec- 

 tive tissue slightly laterad of the postrostrum 

 (fig. 19). This muscle, a second antennal segment 

 promoter muscle, runs anteriorly and laterally to 

 insert in what appears to be a free apodeme just 

 beneath the dorsal rim of the coxopodite. This 

 apodeme is not connected to the coxopodite, but 

 instead (fig. 19) consists of a transverse fascia 

 in which the distal second antennal segment pro- 

 motor muscle originates. The presence of the free 

 apodeme of the proximal second segment pro- 

 motor muscle may indicate that the muscle is in 

 reality a first antennal segment (coxopodite) pro- 

 motor. The free apodeme, however, produces a 

 functional second antennal segment (basipodite) 

 promotor and the muscle is therefore so described. 

 The proximal second segment promotor is evi- 

 dently the homolog of the musculus promotor II 

 antennae in Astacus, Pandalus, and Callinectes, 

 although in these forms the muscle clearly at- 

 taches to the dorsal margin of the coxopodite. 



The distal second antennal segment promotor 

 muscle extends the functional connection of the 

 muscle group to the dorsal edge of the second an- 



