60 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



undoubltedly homologous to the musculi laterales 

 thoracoabdominales of Astacus and Pandalus. 



APPENDAGES OF THE GNATHOTHORAX 



The generalized limb of Crustacea is thought to 

 consist of a basal protopodite, made up of a prox- 

 imal coxopodite and a distal basipodite, bearing 

 two rami, the exopodite and endopodite. This 

 simple arrangement has been retained in the pleo- 

 pods of Crustacea, notably the Natantia. The 

 typical gnathothoracic limb of Penaeus, if one can 

 be said to exist, has the basal coxopodite and basip- 

 odite, the latter with a well-developed endopo- 

 dite and a reduced exopodite. The coxopodite of 

 the white shrimp is attached to the limb base by 

 means of diametrically placed condyles, the axis 

 of the condyles varying in accordance with the 

 location and function of the appendage. The 

 basipodite is attached to the coxopodite by di- 

 condyles whose axis is at right angles to that of 

 the coxal condyles. Distally, the endopodite is 

 divided into the typical five articles: the ischiop- 

 odite, meropodite, carpopodite, propodite, and 

 dactylopodite. The coxopodite may give rise to 

 lateral epipodites and in some cases gills. 



The exopodite may be modified to long, frond- 

 shaped filaments (second and third maxillipeds) 

 or reduced to a small finger projecting from the 

 basipodites (fig. 31). The gnathal appendages, 

 excluding the second and third maxillipeds, tend 

 to be modified substantially from the typical plan 

 given above. 



1. MANDIBLES 

 SKELETAL ELEMENTS 



Of the gnathothoracic appendages, the mandi- 

 bles are perhaps the most difficult to understand. 

 For one thing the dorsal and internal manifesta- 

 tions of this and other gnathal segments are coa- 

 lesced or obliterated. In addition, the mandible is 

 complicated by the presence of a true endoskeleton 

 (figs. 37, 38), lying transversely in the gnatho- 

 thorax above the nerve cord and supporting the 

 gastric mill. Nevertheless, Snodgrass ( 1951b) ad- 

 duces strong evidence in support of the evolution 

 of the mandibles from a typical limb on the basis 

 of a comparative study of the skeleton and muscles 

 of the arthropod mandible. 



The sclerotized parts of the mandibles in 

 Penaeus are relatively simple. The strongly 

 sclerotized incomplete tube of the mandible, with 



its ventromedial incisor and molar surfaces and 

 anterior palp, may be seen in ventral view (fig. 

 ■2'.), mandible). The incisor surfaces consist of 2 

 or 3 sharp ridges used in cutting and tearing food, 

 whereas the flat molar surfaces are for grinding. 

 Both kinds of surface are heavily impregnated 

 with a hardening substance, possibly calcium car- 

 bonate, in the form of crystalline stones molded 

 into the cuticle. The stones may be removed by 

 dissection. The tubular part of the mandible, the 

 body, is said to be the coxopodite, whereas the 

 palp is thought to be part of the endopodite (Cai- 

 man 1909). The subcylindrical body of the 

 mandible is distinctly divided into a large basal 

 portion and a distal lobe, immovably connected to 

 the basal part. In the mandible of Anaspides, 

 the basal part is called by Snodgrass (1952) the 

 coxopodite and the distal lobe an endite of the 

 coxopodite. It is interesting to note that the 

 distal gnathal lobe is movable in some arthropods, 

 notably the Diplopoda, and similar to the maxil- 

 lary lacinia of insects. 



The mandibular palp, a part of the endopodite, 

 extends anteriorly from the body of the mandible 

 in the form of a flat, setose lobe shaped to fit 

 around the labrum laterally and anteriorly. The 

 larger part of the broad palp arises from a basal 

 segment, the latter being narrow at its posterior 

 junction with the mandible and broad anteriorly. 



The mandible of Penaeus is attached to the ven- 

 tral skeleton by a medial condyle located at a point 

 on the posterior arm of the epistome just laterad 

 of the labrum (fig. 28), and by thin cuticle be- 

 tween the mandible and the lateral extension of 

 the posterior epistomal arm. The mandible has 

 also a lateral condyle on the side of the carapace. 

 Among taxonomists the external manifestation of 

 this lateral mandibular condyle is known incor- 

 rectly as the hepatic spine. Snodgrass (1951) 

 erroneously shows two mandibular condyles in 

 Penaeus on the laterally spread, posterior bars of 

 the epistome, and does not mention the lateral 

 mandibular condyle. This is curious since, as 

 Snodgrass (1935, 1951b) has shown in earlier 

 work, a significant phyletic series is evident from 

 the study of the arthropod mandible, based upon 

 the evolutional response of the musculature to re- 

 striction of mandibular movements. In the course 

 of the work, Snodgrass makes the fact amply 

 clear that in no case is the site of the primary 

 (lateral) condyle ever to be found on the epis- 

 tome. Rather, the generalized arthropod man- 



