WHITE SHRIMP FROM THE GULF OF MEXICO 



81 



dactylopodite and turn the distal article ventrad, 

 in opposition to the dactylopodite reductor. The 

 same muscle as the dactylopodite productor in 

 Penaeus is found in Pandalus, Astacus, and Gal- 

 linectes. 



DACTYLOPODITE REDUCTOR MUSCLE OF SECOND 

 MAXILLIPED 



Figure 4.1 



Attached between the proximal part of the pro- 

 podite and the dorsal edge of the dactylopodite is 

 the small dactylopodite reductor muscle. Its func- 

 tion is to turn the dactylopodite dorsad, opposing 

 the action of the productor. The muscle is homol- 

 ogous with the dactylopodite reductor of Panda- 

 lus, Astacus. and Callinectes. 



4c. Third Maxilliped 



The third maxilliped is the first accessory feed- 

 ing appendage which lacks the jawlike character- 

 istics of the anterior gnathal limbs. Far more 

 than any of the other appendages, the third max- 

 illipeds function to grasp large food particles 

 passed up by the chelate legs and hold them next 

 to the mouthparts for further reduction in size 

 and for swallowing. Structurally, the third max- 

 illiped is closely similar to the pereiopods. 



SKELETAL ELEMENTS 



The body hemocoel is confluent with that of 

 the third maxilliped by means of a ventral skele- 

 tal foramen whose fringes are strongly sclerotized 

 and from which phragmal apodemes project over 

 the opening. The sternal plate between the 

 foramina is wider at this point than that between 

 the anterior mouthpart foramina. The heavily 

 sclerotized coxopodite articulates with the ventral 

 skeleton by a dorsal, laterotergal condyle and a 

 ventral sternal condyle, the typical situation in 

 the white shrimp limb. The axis through the con- 

 dyles is about 45° from the vertical, with respect 

 to the transverse plane. Since the distal elements 

 are anterior, movements about these coxopodite 

 condyles raise and lower the appendage, as well 

 as promote and remote it. The basipodite articu- 

 lates with the coxopodite by typical dicondylic 

 connections, the axis of the condyles being hori- 

 zontal. Thus the basipodite accounts for most of 

 the depression and levation of the distal elements, 

 an arrangement that is common to the third maxil- 



liped and all of the walking legs. A large, taper- 

 ing, annulated exopodite articulates with the 

 basipodite laterally. 



The ischiopodite is fused to the basipodite im- 

 movably, although the line of demarcation is clear. 

 The ischiopodite is the longest article of the third 

 maxilliped. The meropodite is connected to the 

 distal end of the ischiopodite by two condyles 

 whose axis permits both flexion of the distal 

 articles towards the midline and reduction of the 

 distal elements. The movements at this joint are 

 extensive. Due to the bending at this joint the 

 shrimp is able to grasp food with the third maxil- 

 liped. The carpopodite articulates with the distal 

 end of the meropodite by two condyles. The axis 

 of these dieondyles is vertical, permitting the 

 carpopodite to flex on the meropodite. 



The axis through the condyles of the joint be- 

 tween the carpopodite and the propodite is also 

 vertical allowing the propodite to be flexed upon 

 the carpopodite. The small dactylopodite is simi- 

 larly articulated with the propodite. 



The distal elements, beginning with the ischio- 

 podite, bear rows of stout spines on their medial 

 sides for holding food particles. The exopodite 

 lias long, plumose setae. The third maxilliped 

 has a branchial arrangement similar to that of 

 the first three walking legs. From the pleural 

 plate arises a pleurobranchia. Two arthro- 

 branchiae project from the dorsal articular mem- 

 brane, and a bilobed mastigobranchia arises from 

 the coxopodite. 



MUSCLE ELEMENTS 



The musculature of the third maxilliped of 

 Penaeus is typical of that seen in the walking legs. 

 Some variation, however, is evident when the third 

 maxilliped of different crustaceans is compared, 

 especially in the musculature of the exopodite and 

 the distal articles of the endopodite. In Penaeus, 

 most of the basipodite and coxopodite muscles 

 originate on the laterotergal, pleural plates rather 

 than on phragmal elements as in the anterior 

 gnathal appendages. Pleural origins of these 

 muscles are typical of all the posterior append- 

 ages, thoracic and abdominal alike. 



The third maxilliped of Penaeus is operated by 

 twenty muscles comprising 12 functional types. 

 That of Astacus contains 17 muscles, including 

 14 types. The third maxilliped of Pandalus is 

 somewhat modified, containing 13 muscles of 9 

 functional types, whereas the same appendage in 



