WHITE SHRIMP FROM THE GULF OF MEXICO 



87 



PROPODITE FLEXOR MUSCLES OF THIRD 

 MAXILLIPED 



Figure 46 



The third maxilliped of Penaeus is unique by 

 comparison with the same appendage of Panda- 

 Jus, Astacus, and Callinectes in having two pro- 

 podite flexor muscles. The larger originates in 

 the carpopodite proximally and passes distally on 

 the lateral side of the article to its insertion on an 

 apodeme of the propodite. The smaller propodite 

 flexor is a little triangular muscle (fig. 46) which 

 has a common insertion with the larger flexor. 

 The flexor muscles turn the propodite mesad on 

 the carpopodite. The larger propodite flexor 

 muscle is found in all the crustaceans referred to. 



DACTYLOPODITE FLEXOR MUSCLE OF THIRD 

 MAXILLIPED 



Figure 46 



The dactylopodite flexor muscle arises in the 

 proximal portion of the propodite and is attached 

 to an apodeme of the dactylopodite. The muscle 

 turns the dactylopodite toward the midline, 

 Dactylopodite flexors appear in the third maxil- 

 liped of Astacus and Callinectes. The distal arti- 

 cles are fused in Panddlus and the muscles thus 

 lost. The third maxilliped of Penaeus has no 

 dactylopodite extensor muscle, a structure de- 

 scribed in Astacus and Callinectes. 



5. PEREIOPODS 



In the Crustacea Decapoda the last five pairs 

 of thoracic appendages are usually referred to as 

 walking legs, or pereiopods. Although their 

 length, size, and functional modifications are var- 

 iable in the group, the walking legs are all funda- 

 mentally alike in structure. Exopodites, usually 

 small, are either present or absent. The large pro- 

 topodite-endopodite is almost always composed of 

 the typical seven appendage articles, although fu- 

 sion of the basipodite with the ischiopodite has oc- 

 curred in some groups. Some or all of the pereio- 

 pods may be chelate. Incidentally, Dougherty 

 (Steinberg and Dougherty, 1957) objects to the 

 spelling of the word "pereiopod" and offers good 

 reasons for dropping the i in American usage. 

 The common and perhaps incorrect spelling is 

 used in the present work. 



In Penaeus, the pereiopods are all relatively 

 long and slender as befits a lightly sclerotized form 

 whose body weight on the walking legs is not 



great. The first three pereiopods bear small che- 

 lae with sharp cutting edges. The last two walk- 

 ing legs are subchelate. Small exopodites are 

 found on all five pairs of pereiopods. The joint 

 between the basipodite and ischiopodite is movable 

 in the walking legs of Penaeus. The coxopodite 

 and basipodite comprise the protopodite to which 

 an endopodite with the typical five articles is at- 

 tached. Associated with the pereiopods are 

 branchiae. With the third maxillipeds, the first, 

 second, and third pereiopods have a bilobed mas- 

 tigobranchia (epipodite) arising from the coxop- 

 odites, 2 arthrobranchiae on the dorsal articular 

 membrane, and 1 pleurobranchia on the latero- 

 tergal plate. The mastigobranchia and one ar- 

 throbranchia is missing on the fourth pereiopod 

 and the fifth pereiopod has only a pleurobranch. 

 Due to pronounced serial similarity of parts in 

 the walking legs, only the first and fifth pereiopods 

 will be considered in detail here. 



5a. First Pereiopod 



The first pereiopod is considered as an example 

 of a chelate limb. It is ordinarily treated as the 

 first walking leg. Whereas in the Reptantia the 

 first pereiopods are modified into huge chelate 

 chelipeds, those of the Penaeidae, while chelate. 

 are similar in size to the remaining four pairs of 

 walking legs. In point of fact, the first pereiopod 

 of Penaeus is not a functional walking leg. That 

 is to say, none of the body weight is supported on 

 the appendage. The first pereiopods are carried 

 horizontally, directed anteriorly, ventrad of the 

 third maxillipeds, and function to pass food par- 

 ticles to the latter. 



SKELETAL ELEMENTS 



The strongly sclerotized coxopodite articulates 

 with the ventral skeleton by dorsoventral dicon- 

 dyles. The axis through the condyles is such that 

 when the coxopodite swings forward it also ap- 

 proaches the midline. The rearward motion, on 

 the other hand, turns the coxopodites away from 

 the midline. The angular attitude of the dicon- 

 dylic axis of the first pereiopod is thus raised 

 laterally with respect to the frontal plane and 

 rostrad with respect to the transverse plane. Pro- 

 ceeding caudad, the angular attitude of the axis 

 through the coxopodite condyles rises laterally 

 with respect to the frontal plane in conjunction 

 with the shift dorsad of the dorsal condyle and 

 the increased width between the ventral sternal 



