44 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



cerebral ganglia. Eacli nerve enters the sac of 

 the labrum and innervates only those sensoria and 

 muscles in half the labrum from the medium 

 sagittal plane laterad. In Penaeus no nerve can 

 be seen crossing the median sagittal plane. Fur- 

 thermore, the embryonic labrum of many higher 

 arthropods (Johannsen and Butt, 1941; Young 

 1953) develops as a paired structure, with paired 

 coelomic sacs. 



SKELETAL ELEMENTS 



Figures 26, 27 



In Penaeus setifems the labrum is a soft, lightly 

 sclerotized sac attached between the widely spread 

 posterolateral bars of the epistomal Y ( fig. 28, .4 ) . 

 It may be noted in passing that these lateral epis- 

 tomal bars are morphologically anterior, al- 

 though due to rearrangements of the head seg- 

 ments, discussed above, the position of the epi- 

 stome is reversed in the head of many crustaceans. 

 The anterior or medial bar of the epistome, 

 dividing the antennal foramina, curves anterodor- 

 sally to form a deep ventral pit, best seen in lateral 

 view (figs. 28, A ; 30) of a cleared anterior skeleton 

 cut along the median sagittal plane. To the epis- 

 tomal invagination, or apodeme, is attached a 

 pair of muscles to be discussed below. 



The labrum is shaped to fit between the antennal 

 bases anteriorly and the incisor and molar sur- 

 faces of the mandibles posteriorly (figs. 26, .4, B). 

 Various auricles and lobes project from the labral 

 surface to enhance its function as an aid in feeding. 

 A toothed structure, the posterior feeding process 

 (figs. 26, 27), projects directly into the mouth 

 aperture. 



MUSCLE ELEMENTS 



EPISTOMAL STATOR MUSCLES 



Figures .">, (5, :M). 34 



Originating on the dorsal surface of the cara- 

 pace, lateral to the posterior protocephalon levator 

 muscles, and converging on the anterior side of the 

 epistomal invagination is a pair of small muscles 

 which are named in the present work the epistomal 

 stator muscles (figs. 5, 6). The name derives from 

 the fact that contractions of the muscles would 

 appear to hold the epistomal invagination in posi- 

 tion during the contraction of other muscles in the 

 area. The epistomal stator muscles are homolo- 



gous with the musculus oculi basalis posterior in 

 Pandalm, Astacus, and probably in Callinectes. 

 Giving the name, epistomal stators, to these 

 muscles may be adding confusion to the morpho- 

 logical scene, since these muscles are undoubtedly 

 the musculus attractor sincipitis described by 

 Grobben (1919) in the stomatopod Squilla mantis 

 and the muscle attracteur du synciput illustrated 

 by Mayrat (1955, 1956a, 1956b) in Praunus fle.r- 

 uosus O. F. Midler. The "sincipit" (Mayrat spells 

 it "synciput") of Grobben ( 1917) is of course the 

 protocephalon of Snodgrass (1951) as applied to 

 the Crustacea. No great objection is offered here 

 to designating the muscles in Squilla and Praunus 

 as the synciput attractors. The muscles are in- 

 deed synciput or protocephalon muscles appar- 

 ently functioning in certain forms to draw the 

 protocephalon posteriorly. However, the same 

 muscles in Penaeus do not attract the protocepha- 

 lon. Furthermore they insert on a specific region 

 of the protocephalon, the epistome, and so deserve 

 as special a name as possible. The problem can 

 be resolved by a study of the nerves, for if the 

 epistomal stator muscles belong to the epistome, 

 they should be innervated by epistomal or clypeal 

 nerves. 



LABRAL MUSCLES 



Figure 27 



One of the most astonishing features of the 

 anatomy of Penaeus setiferus is the musculature 

 of the labrum. In the generalized insect labrum, 

 the structure is moved by two pairs of extrinsic 

 muscles, the anterior and posterior labral muscles 

 arranged for production and reduction. To the 

 intrinsic labral compressor muscles of insects may 

 be assigned various functions. In contrast, a re- 

 view of general and special accounts of the anat- 

 omy of Crustacea has shown no reference to labral 

 muscles in this class. Yet the labrum of Penaeus 

 (fig. 27) is operated by at least 12 pairs of in- 

 trinsic muscles, bilaterally situated, and at least 

 1 intrinsic muscle running across the entire lobe. 

 From their arrangement, the labral muscles ob- 

 viously distort the labrum in all sorts of ways in 

 the function of the organ as a tongue. In addi- 

 tion, at least 2 pairs of extrinsic muscles insert on 

 the edge of the labral foramen to move the entire 

 organ. No attempt has been made here to assign 

 functional muscle names to the individual labral 

 muscles. 



