WHITE SHRIMP FROM THE GULF OF MEXICO 



125 



muscle. Like the distal exopodite extensor mus- 

 cle, the distal exopodite flexor muscle is a homo- 

 log of the exopodite flagellum muscle (musculus 

 flagellaris exopoditis III pedii spurii) in Astacus, 

 Cambarus, and Pandalus. 



ENDOPODITE MUSCLE 



Figure 68 



The endopodite muscle, or muscles, appears to 

 be a multipart structure in Penaeus setiferus. A 

 portion of the muscle lies mesially in the antero- 

 distal lobe of the basipodite (fig. 68). Fibers of 

 the muscle run through the constricted article con- 

 necting the basipodite with the endopodite and 

 continue distally into the endopodite. Additional 

 fibers that arise in the proximal end of the endo- 

 podite suggest that the endopodite contains more 

 than one muscle body. The function of the endo- 

 podite musculature consists of stiffening the endo- 

 podite during the power stroke and, by their relax- 

 ation in the recovery stroke, of enabling the endo- 

 podite to bend before the flow of water. 



The endopodite flagellum muscle of Astacus, 

 Cambarus, and Pandalus is very likely the homo- 

 log of the muscle in the pleopod endopodite of 

 Penaeus. The muscle has been lost in the blue 

 crab. 



2. Tail Fan 



The tail fan is made up of the telson and uro- 

 pods projecting from the posterior end of the sixth 

 abdominal segment and intimately associated with 

 it. Classically, the uropods have been treated as 

 serially homologous appendages having the typi- 

 cal limb parts, however modified. The telson, on 

 the other hand, has usually been considered an un- 

 segmented posterior element bearing the anus, on 

 grounds of the embryological addition of abdomi- 

 nal segments before the telson (Caiman 1909). 

 The whole tail fan is well adapted for the purpose 

 of drawing the white shrimp backwards through 

 the water as the great ventral abdominal muscles 

 flex the abdomen. 



SKELETAL ELEMENTS 



The telson (figs. 70 to 74) is an apparently 

 unpaired structure. Its broad, anterior portion 

 articulates with the caudal end of the postero- 

 dorsal part of the sixth abdominal somite. Lateral 

 condyles allow free movements in the vertical 

 plane, but limit horizontal motion. The telson 



468059 0—59 9 



becomes narrow posteriorly, tapering to a sharp 

 point. In section, the telson is roughly triangular. 

 The thin sternum varies in shape from flat to 

 slightly concave and the heavily sclerotized latero- 

 tergal plates are convex. Instead of having a 

 mid-dorsal carina or ridge, the telson has a pair 

 of ridges produced by a shallow groove in the 

 dorsal midline (fig. 70). The structure lies above 

 parts of the uropods and affords them some pro- 

 tection dorsally. 



The uropods (figs. 70 to 7-t) arise from the 

 posteroventral area of the sixth abdominal somite. 

 Each is comprised of a strongly sclerotized basal 

 element, the protopodite, from which the broad, 

 flat uropods project. The large lateral uropod is 

 the exopodite while the inner is the endopodite. 

 The protopodite is supposed to be made up of the 

 coxopodite and basipodite, but no skeletal trace 

 of these articles is evident. A faint relic of seg- 

 mentation in the exopodite remains on the dorsal 

 surface (fig. 70) in the form of a transversely 

 oriented groove. A portion of this groove sets 

 off the tip of the lateral exopodite adductor muscle. 



Due to the hinging of the uropodal elements, 

 the uropods are capable of free motion. The artic- 

 ulation between the protopodite and the sixth ab- 

 dominal segment, while strong, is relatively loose. 

 The protopodite can move in the vertical and 

 horizontal planes and also may rotate about its 

 long axis. The points of articulation between 

 the protopodite and the uropods are, on the other 

 hand, condylic, and limit the exopodite and endo- 

 podite to the horizontal movements of spreading 

 the uropods. Certain marginal areas of the telson 

 and uropods are fringed with natatory hairs, much 

 like the pleopods. 



MUSCLE ELEMENTS 



The muscles of the tail fan are all disposed to 

 the function of these organs in the rapid back- 

 ward swimming of the white shrimp. In plan the 

 muscles are widely different from that of the 

 typical limb. The most notable difference is the 

 presence inside the sixth abdominal segment of 

 several muscle groups which operate parts of the 

 tail fan. This does not occur in the preceding 

 abdominal segments. In comparing the muscles 

 of the area in Penaeus with the tail fan muscles of 

 Astacus and Pandalus, two kinds of differences 

 are encountered. The first has to do with the num- 

 ber of functional muscle types, Penaeus having 

 16 compared with 18 muscle types in the crawtish. 



