WHITE SHRIMP FROM THE GULF OF MEXICO 



121 



mesial coxopodite adductor muscle, which has dis- 

 appeared in Callinectes, possibly exists in 

 Astacus, Cambarus, and Pandalus as the ventral 

 basipodite rotator muscle (musculus rotator ven- 

 tralis basipoditis III pedis spurii). From Berke- 

 ley's account of the origin, insertion, and action 

 of the muscle in Pandalus, the musculus rotator 

 ventralis basipoditis is really an adductor muscle. 

 Again, a study of the nerves is called for. 



BASIPODITE ABDUCTOR MUSCLE 



Figure 68 



Opposing the action of the coxopodite adductor 

 muscles which tend to draw the basipodite and 

 other distal parts of the pleopod towards the ab- 

 domen is the basipodite abductor muscle. This 

 fan-shaped muscle originates on the mesial sur- 

 face of the basipodite. The area of origin is elon- 

 gate in the long axis of the basipodite (fig. 68). 

 The muscle becomes narrow as it courses dorso- 

 laterally to insert on the ventrolateral margin of 

 the coxopodite. When the basipodite abductor 

 muscle contracts, the basipodite and the distal 

 pleopod elements are drawn away from the ab- 

 domen. The muscle has been lost in Callinectes 

 and in the Astacura referred to above. The basi- 

 podite abductor muscle of Penaeus may have a 

 homolog in Pandalus as the musculus adductor 

 basipodite II pedis spurii. a muscle that, according 

 to the description of Berkeley, is evidently an 

 abductor. 



BASIPODITE PROMOTOR MUSCLE 

 Figures 65, 66, 67, 69 



The basipodite promotor muscle originates on 

 an area of the anterior margin of the basipodite 

 about one-third of the distance from the distal 

 end of the basipodite. The muscle inserts on the 

 anteroventral margin of the coxopodite. Muscle 

 contractions draw the basipodite, and the distal 

 parts of the pleopod, cephalad with respect to the 

 coxopodite. Together with the coxopodite pro- 

 motor, the basipodite promotor muscle moves the 

 pleopod in its recovery stroke. The basipodite 

 promotor extends the length of the stroke. No 

 homolog of the basipodite promotor muscle in 

 Penaeus is evident in the pleopod of Astacus, 

 Cambarus, or Callinectes. 



The third pleopod of Pandalus, however, has 

 a muscle, the musculus productor basipoditis III 



pedis spurii, that is similar in function and ar- 

 rangement to the basipodite promotor muscle of 

 Penaeus. The muscles in the two animals are al- 

 most certainly homologous, although proof must 

 wait upon a study of nerve-muscle connections. 

 Berkeley's use of the term "productor" for this 

 muscle is questionable. She says, in part, that the 

 musculus productor basipoditis ". . . moves the 

 basipodite forward. . . ." In the opinion of the 

 present writer, the action described by Berkeley 

 for the muscle is not production, but promotion. 

 Production, in the present context, is synonymous 

 with the extension in length of a limb, and the 

 musculus productor basipoditis of the third pleo- 

 pod of Pandalus does not appear to be an extensor 

 muscle. 



BASIPODITE REMOTOR MUSCLES 

 Figures 65 to 69 



The pleopod of the white shrimp contains 3 

 basipodite remotor muscles. Their contractions 

 serve to increase greatly the length and power 

 of the propulsive stroke initiated by the coxopo- 

 dite remotor muscle. The anterior basipodite re- 

 motor muscle originates over a broad area in the 

 anteroventral lobe of the basipodite (figs. 65, 66). 

 From its origin the muscle runs diagonally across 

 the interior of the basipodite to insert on the 

 ventral edge of the coxopodite, posterior to a 

 line through the points of articulation between 

 the basipodite and the coxopodite. The posterior 

 basipodite remotor muscles, of which there are two 

 (figs. 65, 68), originate in the posteroventral lobe 

 of the basipodite and extend dorsad to insert on 

 the ventral margin of the coxopodite postaxially. 



On the basis of arrangement and size, the an- 

 terior basipodite remotor muscle of Penaeus is 

 evidently a homolog of the musculus reductor 

 basipoditis III pedis spurii of Pandalus. The 

 muscles are also functionally similar. Here again 

 a question is raised over the use of terms. Berke- 

 ley describes the action of the musculus productor 

 basipoditis as antagonistic to that of the muscu- 

 lus reductor basipoditis. If, as has been estab- 

 lished above, the name "productor" for muscle 

 function is synonymous with the term "extensor," 

 then it follows that a reductor muscle is the same 

 as a flexor muscle. Unfortunately, reduction has 

 not in the anatomical sense the opposite meaning 

 of production, but implies additional functions. 

 For this reason the name "reductor" for muscle 



