WHITE SHRIMP FROM THE GULF OF MEXICO 



43 



groove of the fifth antennal segment. Contrac- 

 tions of the fifth segment promoters move the fifth 

 antennal segment anteriorly in a limited way. 

 The homolog of these muscles in Astacus is 

 very like the musculus extensor propoditis II an- 

 tennae. Berkeley (1928) does not find the muscle 

 in PandaHus. 



FLAGELLUM EXTENSION MUSCLES 

 Figure 24 



The fifth antennal segment of Penaeus contains 

 two flagellum extensor muscles (fig. 24), occupy- 

 ing the medial half of the segment. One of the 

 flagellum extensors originates on the medial side 

 of the fifth segment and runs distally to insert on 

 the large extensor apodeme on the medial side of 

 the flagellum base. The second extensor takes ori- 

 gin on the posteroventral region of the fifth seg- 

 ment, runs distally, and inserts on the large exten- 

 sor apodeme on the flagellum. These muscles 

 bring the base of the flagellum directly anterior to 

 the proximal antennal segments. One or both of 

 these muscles is undoubtedly homologous with the 

 antennal dactylopodite extensor muscle as shown 

 by Schmidt (1915) in Astacus and by Berkeley 

 (1928) in Pandalus. The related muscle, if any, 

 in CaUinectes is uncertain without adequate in- 

 formation about the nerves. 



FLAGELLUM FLEXOR MUSCLES 



Figure 24 



Three flagellum flexor muscles in the fifth an- 

 tennal segment of Penaeus turn the antennal fla- 

 gellum to its normal position at right angles to the 

 proximal antennal segments. The largest and 

 ventral-most of these muscles originates broadly 

 along the proximal groove of the fifth segment and 

 inserts on the flexor apodeme on the flagellum 

 base. Dorsal to the large muscle, two flagellum 

 flexor muscles insert on the same flexor apodeme 

 on the flagellum. The dorsolateral flagellum flex- 

 or originates in the lateral corner of the fifth seg- 

 ment, the dorsomedial flexor originating in the 

 medial corner of the fifth antennal segment. At 

 least one of these muscles in Penaeus is the homo- 

 log of the musculus flexor dactylopoditis II anten- 

 nae in Astacus and Pandalus. Whether homo- 

 logs exist in CaUinectes is not known. 



4. LABRUM 



The labrum is the final component of the pro- 

 toeephalon to be considered. In all arthropods the 



labrum is a lobe or sac suspended over the mouth 

 from a sclerotized region of the head known as the 

 epistome (the hexapod clypeus) . Crustacean mor- 

 phologists ordinarily do not consider the labium 

 an appendage. Most workers consider the labrum 

 an unpaired structure. Some students of Crus- 

 tacea place the epistome as the ventral element of 

 a preoral, premandibular segment, behind the eyes, 

 antennules, and antennae, in the order of their oc- 

 currence in many adult crustaceans. Others have 

 even assigned the epistome to the sternum of the 

 antennal or mandibular segments, giving the lab- 

 rum an utterly indefensible postoral position. 



The position of the epistome and labrum in 

 adult arthropods is variable. In some crusta- 

 ceans, like the adult isopods and amphipods, and 

 in most insects, the epistome is anterior or facial 

 (Snodgrass 1951 ) . The labrum thus is ventral or 

 anterior to it. However, in most crustaceans, 

 some chilopods, and a few insects the head seg- 

 ments have thrust forward, overgrowing the 

 epistome anteriorly. The result is a secondary 

 ventral position of the epistome and labrum in 

 some arthropods. In point of fact, the labrum is 

 the anterior end of the arthropod. I am in full 

 agreement with Snodgrass (1951) in this view. 

 Furthermore, the labrum is here considered not 

 only the most anterior part of the arthropod, but 

 also the anterodorsal "upper lip" of these forms, 

 and as such the dorsal part of the first segment. 

 Considerable support for this interpretation has 

 been adduced by Ferris (1947) and Henry 

 (1948a), based on the study of the labral nerves. 



If we accept the view of various workers, in- 

 cluding Henry (1948a), that the tritocerebrum of 

 arthropods is in reality the first ganglion of the 

 ventral nerve cord, whatever its fate in the adult, 

 then we are bound to regard structures innervated 

 by tritocerebral nerves as primitively anterior. 

 Since the labrum of Penaeus is clearly innervated 

 by a pair of nerves from the tritocerebral ganglia 

 (figs. 27, 7fi), similar to the situation in other 

 Crustacea and Insecta (Henry 1948a, 1948b), then 

 the labrum is segment 1 in Penaeus. 



Xo evidence is here advanced to suggest that 

 the labrum is a reduced appendage. Indeed, this 

 ancient "upper lip" was probably never an arthro- 

 appendage in the true sense at any time in its 

 history. The labrum is, however, very likely 

 paired. In support of this is the morphology of 

 the nerves and muscles. The labral nerves are 

 paired and arise from the clearly bilateral trito- 



