WHITE SHRIMP FROM THE GULF OF MEXICO 



15 



of cuticular material. Slightly anterodorsal to 

 the antennular apodeme, in the same parasagittal 

 plane, is an apodeme which invaginates from the 

 ventral floor of the basal segment of the eyestalk 

 and projects through the ventrolateral side of the 

 eyestalk foramen into the thoracic hemocoele. 

 This apodeme, like that near the antennule, 

 broadens vertically. By virtue of apodemal posi- 

 tion, that part of the protocephalon attractor 

 muscle attaching upon the eyestalk apodeme is 

 somewhat longer than is the part inserting on the 

 antennular apodeme. 



The longest and most dorsal part of the proto- 

 cephalon attractor muscle extends anteriorly be- 

 yond the antennular and eyestalk apodemes, to in- 

 sert slightly laterad in connective tissue at the 

 ventral surface of the basal segment of the eye- 

 stalk (figs. 7, 8). 



To these comparatively huge protocephalon 

 muscles we may ascribe at least two functions, 

 namely, ( 1 ) attraction of the protocephalon, and 

 (2) adduction of the eyes. The largest of the three 

 inserting bodies of the muscle is the ventral-most 

 part inserting on the antennular apodeme, de- 

 scribed above. From a study of the points of ar- 

 ticulation between the carapace and I lie proto- 

 cephalon. dorsally, and the mandibular segment 

 and the protocephalon, ventrally, the primary re- 

 sult of contraction of the ventral muscle body 

 woidd be to draw the protocephalon directly pos- 

 terior. The muscle was described as a proto- 

 cephalon depressor (musculus depressor sincipitis) 

 by Grobben (1!>17) in a number of Decapoda 

 and in the stomatopod Squilla. This worker has 

 shown that the protocephalon attractors, or de- 

 pressors, are missing in those forms, like Astacus, 

 in which the protocephalon is immovably fused 

 to the thorax. The point of Grobben's discussion, 

 that the protocephalon is movable on the gnathal 

 tagma in the generalized crustacean, is not 

 changed by a difference of opinion over the func- 

 tion of the muscle under consideration. 



Furthermore, these muscles are not antennular 

 in any way, having, as brought out by Snodgrass 

 (1!);")]), origins on the carapace. They are also, 

 to say the least, not strictly antenna], since a por- 

 tion of the muscle inserts in the eyestalk. Al- 

 though the protocephalon attractor muscles in 

 Penaeus brasiliensis Latreille, 1817, may be widely 

 different from those in P. setiferus, the state- 

 ment of Knowles (1953) that the two muscles ly- 

 ing just beneath the anterolateral side of the 



carapace are antenna] is probably in error. From 

 his figures the external-most muscle is properly a 

 remotor of the antennal scale, while the large in- 

 ner muscle is the protocephalon attractor muscle. 



The two anterodorsal parts of the protocephalon 

 attractor muscles which find insertions in the eye- 

 stalks have, in addition to attraction, the function 

 of eyestalk adduction. Upon contraction of the 

 whole muscle, these dorsal fibers in the eyestalk 

 draw the ocular plate and attached eyestalk seg- 

 ments posteriorly toward the carapace. The pos- 

 terior side of the basal segments makes contact 

 with a condylic thickening on the anterior edge of 

 the carapace, at a point known as the orbital angle 

 ( fig. 4) , thereby swinging the eyestalks forward in 

 a horizontal plane into the ocular depressions on 

 the antenlnules. As suggested in introductory 

 comments, the movement is a quick one, much 

 faster than the return of the eyes to the spread 

 position. In Penaeus setiferus other muscles exist 

 which function to ackluct the eyes, but their effect 

 is negligible when compared to that of the much 

 larger protocephalon attractor muscles. 



The protocephalon attractor muscles appear in 

 Pandalus, designated by Berkeley (1928) as the 

 depressor muscles c of the antennae, on grounds 

 of their attachment to the basipodites of those 

 structures. At the same time this worker ascribes 

 to the depressor muscles c the function of adduc- 

 tion and rotation, rather than the depression of 

 the antennae. Berkeley's name for the muscles 

 obviously was taken from the work of Schmidt 

 (1915) on Astacus, in which form the antennal 

 depressor muscles c, while small, nonetheless de- 

 press the antennae. Although proof must wait 

 upon a study of the nerves in Penaeus and Pan- 

 dalus, Berkeley has homologized the so-called de- 

 pressor muscle c of Pandalus and Astacus on the 

 basis of their dorsolateral origins on the cara- 

 pace and their insertions on the mediodorsal edge 

 of the antennal basipodite (in Pandalus) and 

 coxopodite (in Astacus). 



That the depressor muscles c in Pandalus 

 and the protocephalon attractors in Penaeus are 

 homologous seems fairly certain, despite the 

 apparent change of insertion in the former. A 

 review of cleared and stained exoskeletons of 

 Pandalus might show multiple insertions of the 

 muscle as in Penaeus. The homology of the pro- 

 tocephalon attractor muscles in Peneaus with the 

 depressor muscles c in Astacus is less certain. 

 In Callinectes, Cochran (1935) figures a pair of 



