WHITE SHRIMP FROM THE GULF OF MEXICO 



39 



in I'n mhi) 'us. However. Berkeley illustrates a dis- 

 tal scale muscle which she designates as the mus- 

 cuhis adductor exopoditis b II antennae and 

 which from the standpoint of its arrangement ap- 

 pears to be the distal scale abductor muscle in 

 Penaeus. A careful review of the insertion of 

 Berkeley's exopodite adductor muscle might show 

 that it is in reality an abductor muscle. 



SCALE ADDUCTOR MUSCLES 



Figures 20. 21. 24 



Two types of scale adductor muscles (tigs. 20. 

 21, 24) are found in the antenna of Penaeus. At 

 least two scale adductors originate on the medial 

 wall of the second antennal segment ventral to the 

 excretory pore (fig. 24, B), and run diagonally to 

 insertion points on the ventral and medial margins 

 of the scale foramen. Their insertions are mesad 

 of the axis of the scale condyles. The distal scale 

 adductor muscle (tig. 24, B), like the distal scale 

 abductor, is located in the body of the scale. It 

 originates in the distal region of the exopodite 

 and runs caudad parallel to the distal scale abduc- 

 tor to insert on the margin of the basipodite fora- 

 men mesad of the scale articles. Upon contraction, 

 the scale adductor muscles move the antennal scale 

 inward toward the median line of the shrimp, in 

 opposition to the action of the scale abductors. 



THIRD ANTENNAL SEGMENT ROTATOR MUSCLE 



Figures 20, 21, 24 



Arising on the dorsomedial rim of the first an- 

 tennal segment (coxopodite) foramen and running 

 ventro-medially, the third antennal segment rota- 

 tor muscle (figs. 20. 21, 24) inserts on an apodeme 

 located on the lateral margin of the third antennal 

 segment ( figs. 23, 24) . Contractions of the muscle 

 accomplish the movement described in the section 

 on the skeletal elements, namely, lateral rotation 

 of tlie third antennal segment and the antennal 

 parts distal to the third segment. 



The homologs of this muscle in the other 

 crustaceans referred to are not clear. The best 

 possibility in Astaeus is one or both of the mero- 

 podite muscles a and b. On the basis of area of 

 insertion, the most likely homologs is the mus- 

 culus meropoditis it. although the third segment 

 rotator in Penaeus and the meropodite muscle a 

 in Astaeus have different origins, the latter being 

 in the basipodite of the crawfish antenna. The 



meropodite muscle in Pandalus is more nearly 

 similar to the situation in Penaeus than in Asta- 

 eus. In both Astaeus and Pandalus, Schmidt 

 (1915) and Berkeley (1928) illustrate a muscle, 

 the musculus reductor ischiopoditis II antennae, 

 said to oppose the action of the meropodite 

 muscles a and b. No similar muscle has been 

 found in Penaeus, although the shrimp may have 

 a functional analog in the fourth antennal segment 

 adductors. 



FOURTH ANTENNAL SEGMENT ADDUCTOR 

 MUSCLE 



Figure 24 



The fourth antennal segment adductor muscle 

 (fig. 24) originates in a broad fan on the ventral 

 surface of the basipodite. The muscle runs an- 

 terodorsally. narrowing to its apex at its point 

 of insertion on a small, movable article presumed 

 to be a part of the fourth antennal segment. 

 Certainly a muscle originating on the basipodite 

 and inserting on the fourth antennal segment is 

 unusual. Controversy could be avoided by as- 

 signing the small, movable article to which this 

 muscle inserts to the third antennal segment : 

 however, the movable article appears to be widely 

 separated from the third segment and instead is 

 lateral to the fourth segment and clearly con- 

 nected to it. When the fourth segment adductor 

 muscle contracts, the movable article is drawn 

 posteroventrally with the result that the fourth 

 antennal segment is rotated upon the third seg- 

 ment and turned a short distance mesad. As 

 such, the fourth segment adductor represents in 

 part an opposing muscle to the third segment 

 rotator muscle. The homology of this muscle is 

 uncertain. 



FOURTH ANTENNAL SEGMENT ABDUCTOR 

 MCSCLE 



Figure 24 



Originating from a broad area slightly poste- 

 rior to the origin of the fourth segment adductor 

 muscle, the fourth antennal segment abductor 

 muscle (fig. 24) runs to an insertion on the same 

 small movable article of the fourth antennal seg- 

 ment to which the fourth segment adductor at- 

 taches. The fourth segment abductor is much 

 larger than the fourth segment adductor. Upon 

 contraction the fourth antennal segment abductor 

 reinforces the action of the fourth segment ad- 



