WHITE SHRIMP FROM THE GULF OF MEXICO 



115 



thoracic muscles. No posterior oblique muscle has 

 been identified in the highly modified sixth ab- 

 dominal segment of Penaeus. The posterior 

 oblique muscles of Penaeus are represented in the 

 abdomen of Astacus and Pandalus. The latter 

 forms also have five posterior obliques. 



OBLIQUE TRANSVERSE MUSCLES OF ABDOMEN 

 Figure 61 



The oblique transverse muscle splits off the an- 

 terior oblique muscle clorsomedially at the junction 

 of the main anterior oblique muscle and the ex- 

 ternal arm. The fibers turn ventrad between the 

 anterior oblique and the central muscle and there 

 join the central muscle. The affinities of the 

 oblique transverse muscles of Penaeus in Astacus 

 and Pandalus are not known. 



1. Pleopods 



The pleopods or swimming legs in many natant 

 Crustacea are well -developed swimming append- 

 ages that enable the animals to propel themselves 

 forward rapidly for great distances. In Penaeus 

 setiferus the pleopod is not only heavily muscled 

 but so constructed that the muscles move the pleo- 

 pod, and in particular the propellers, the exopo- 

 dite and endopodite, through a long power stroke. 

 Each pair of pleopods beats in unison. The beat 

 of the pleopods is synchronized by volleys of nerve 

 impulses passing along the ventral abdominal 

 nerve cord so that a beat wave reminiscent of 

 ciliary action passes down the abdomen from an- 

 terior to posterior. P. setiferus has five pairs of 

 pleopods, typical of the Crustacea Xatantia. The 

 third set has been chosen for study here because 

 of their unspecialized structure. 



When feeding, the white shrimp creeps along 

 the bottom on its long, slender walking legs, test- 

 ing the substrate for food particles. The cephalic 

 region of the shrimp is held high while the tail fan 

 and telson rest on the bottom. As the animal 

 moves about, the pleopods beat gently from time 

 to time. The resulting flow of water aids in the 

 animal's progress by slightly lifting the tail fan 

 from the bottom. Close examination of the white 

 shrimp on the bottom reveals that the main propul- 

 sive elements, the exopodites, beat in a lateral posi- 

 tion, not brushing the substrate. The present 

 writer assumed that when the shrimp, encounter- 

 ing an obstruction, rises free of the substrate, the 



pleopod exopodites would beat in a vertical plane, 

 in a straight line with the proximal elements of 

 the pleopods. Such is not the case, however. In 

 P. -setiferus the pleopod exopodites always beat in 

 a horizontal plane at right angles to the proximal 

 pleopod elements during the power stroke, no mat- 

 ter whether the animal is creeping along the bot- 

 tom or swimming freely in the water above. Dr. 

 Edward Peebles, Tulane Medical School, has sug- 

 gested that the lateral position of the pleopod exo- 

 podites places these organs outside of the stream of 

 turbulence created by the walking legs. 



The return or recovery stroke, during which the 

 pleopod exopodite must be feathered, is made in 

 the vertical plane, or nearly so. This action might 

 be thought to cause the long exopodite to drag on 

 the bottom, but during the return stroke the organ 

 is relaxed and bends before the pressure of the 

 water passing over it. The distal tip of the exo- 

 podite describes an oval. The power stroke draws 

 the tip in a flat arc lateral to the ventral plane of 

 the abdomen. At the end of the power stroke, wa- 

 ter pressure from the anterior causes the exopo- 

 dite to bend caudally and, together with the con- 

 traction of rotator and flexor muscles, to rotate 

 one-quarter of a turn about its longitudinal axis. 

 Upon rotation of the exopodite, the inertial drift 

 of the animal through the water helps extensor 

 muscles bring the exopodite from the lateral posi- 

 tion of the power stroke to a ventral position. Ro- 

 tation of the exopodite also enables the organ to 

 present to the flow of water its cross section of least 

 resistance as it is brought forward in recovery. 



SKELETAL ELEMENTS 



The skeletal and muscular elements of the third 

 pleopod of Penaeus setiferus are arranged to per- 

 form the functions described above. The skeletal 

 parts are roughly similar to those of the third 

 pleopod of the European crawfish Astacus astacus, 

 to those of the first pleopod (the pleopod of the 

 second abdominal segment) in the female of Cal- 

 linectes sapidus, to those of the third pleopod of 

 Cambams longulus longulus, and to the parts of 

 the third pleopod of Pandalus danae. 



Where the third pleopod of Penaeus setiferus 

 is attached to the abdominal venter, the region of 

 articulation of the ventral skeleton is reinforced 

 by two V-shaped structures comprised of sclero- 

 tized bars (fig. 59). The lateral V (fig. 65, ven- 

 tral skeletal support), traditionally said to be a 

 contribution of the pleuron, has its apex directed 



