64 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



gnathal elements of the mandibles. These two 

 mandibular abductor muscles are fully homolo- 

 gous with the major and minor mandibular ab- 

 ductor muscles in Astacus and with the mandibu- 

 lar abductor muscle in Pandalus. Presumably 

 the homology holds for the major and minor 

 mandibular abductors of Callinectes. 



DORSOVENTRAL MANDIBULAR TENSOR MUSCLE 



Figure 37 



The long tendon of the dorsoventral mandibular 

 tensor muscle (fig. 37) originates in connective 

 tissue on the dorsal carapace by an extremely thin 

 band mesad of the origin of the antennal promotor 

 muscle. The thin apodeme runs ventrad to the 

 muscle body which inserts on the anterior part of 

 the mandibular endoskeleton, slightly laterad of 

 the small mandibular abductor muscle. The mus- 

 cle pulls the mandibular endoskeleton dorsad, pos- 

 sibly as a minor adjustment of the mandible in 

 feeding. The muscle was named by Grobben 

 (1917), who described it in a number of crusta- 

 ceans, including species of Penaeus, Palaemon, 

 Leander. Pandalus, and Nebalia. The description 

 by Berkeley (1928) of Pandalus danae does not in- 

 clude the dorsoventral mandibular tensor muscle, 

 although it is almost certainly present in that 

 species. The muscle is missing in Astacus. 



ANTERIOR MANDIBULAR ADDUCTOR MUSCLES 

 Figures 33, 34, 37, 38 



The anterior mandibular adductor muscles (figs. 



33, 34, 37, 38) are the largest occupants of the 

 subcylindrical mandibular body. At least three 

 muscles are evident in Penaeus, although a study 

 of the nerves may show that the muscle groups 

 are actually parts of the same muscle. The adduc- 

 tors originate laterally throughout the body of 

 the mandible and insert extensively over the tis- 

 sues of the mandibular endosternite. Contractions 

 of the anterior mandibular adductors draw the 

 gnathal lobes of the mandibles together. The ac- 

 tion is direct and efficient. The anterior mandibu- 

 lar adductor muscles of Penaeus are the homo- 

 logs of the musculus adductor anterior mandibu- 

 lae in Astacus and Pandalus. 



DORSOVENTRAL MANDIBULAR LIGAMENT 

 Figures 33, 34, 35, 37 



The dorsoventral mandibular ligament (figs. 33, 



34, 35, 37) originates in the thick connective tissue 



of the dorsal carapace by a narrow, thin, apodeme 

 between the dorsal edges of the protocephalon at- 

 tractor muscle and the gastric mill (fig. 35). The 

 apodeme runs anteroventrally to a thin, fan- 

 shaped muscle inserting on the dorsal surface of 

 the endosternite. It crosses over the tendon of the 

 dorsoventral maxillary tensor muscle medially. 

 The muscle is closely appressed to the fibers of the 

 dorsoventral maxillary tensor, but may be sep- 

 arated readily from the latter in the sagittal 

 plane. The dorsoventral mandibular ligament (or 

 muscle) aids in retaining the endosternite in 

 position. 



The structure was found in a species of Penaeus 

 and of Palaemon and named by Grobben (1917). 

 Since the dorsoventral mandibular ligament ac- 

 tually ends in a small muscle, Grobben's name 

 should be replaced with the name, "dorsoventral 

 mandibular muscle." A muscle in Pandalus, des- 

 ignated by Berkeley (1928) as the musculus dorso- 

 ventralis anterior 2, is probably the dorsoventral 

 mandibular ligament. 



POSTERIOR MANDIBULAR ADDUCTOR MUSCLE 



Figures 30, 33, 34, 35, 37, 38 



Taking origin in an elongate ovoid on the dorsal 

 carapace is a large, wedge-shaped muscle, the pos- 

 terior mandibular adductor muscle (figs. 35, 37). 

 Growing narrower as it passes ventrad, the pos- 

 terior adductor attaches to a broad apodeme (figs. 

 29, B; 30, apodeme, mandibular adductor muscle) . 

 The adductor apodeme arises from the posterior 

 margin of the semitubular mandibular body. The 

 apodeme is so placed that a dorsal pull of the pow- 

 erful muscle brings the gnathal lobe to the mid- 

 line. The posterior mandibular adductor muscles 

 are widely represented in the Arthropoda. 



MANDIBULAR PALP FLEXOR MUSCLE 

 Figure 29 



The muscles operating the mandibular palp are 

 situated either in the gnathal lobe of the mandible 

 or in the basal segment of the palp. The distal 

 lobe of the palp contains no muscles. The mandib- 

 ular palp flexor muscle (fig. 29) originates in the 

 proximal region of the basal palp segment near 

 the foramen between the gnathal lobe and the palp 

 base. The muscle runs distally, becoming broad 

 and flat, and inserts on the posteroventrad margin 

 of the distal palp lobe. In action, the mandibular 

 palp flexor turns the distal lobe ventrad. An ap- 



