58 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



The median anterior thoracic muscles (fig. 36) 

 also originate in segmentally arranged fasciae on 

 the ventral surface. Joined by the other anterior 

 thoracic muscles, the median anterior thoracic 

 muscles pass caudad into the abdominal muscles. 



The anterior thoracic muscles play an impor- 

 tant part in the powerful flexion of the abdomen 

 on the thorax made by the white shrimps when 

 the animals withdraw suddenly from danger. 

 The anterior thoracic musculature is similar to 

 that in Pandalus and Astacus, except that in the 

 crawfish these muscles are somewhat smaller. 

 The anterior thoracic muscles are fully homolo- 

 gous in all the forms mentioned here. 



VENTRAL HEAD LIGAMENTS 



Figures 37, 38 



The ventral head ligaments are small structures 

 attached between the lateral wings of the epistome 

 and the mandibular endosternite (fig. 37). Ap- 

 parently they hold the endosternite in position an- 

 teriorly. Schmidt (1915) calls these ligaments the 

 ventral head muscles in Astacus. Grobben (1917) , 

 however, denies the presence of muscle fibers in the 

 structures and suggests the name ligament for mus- 

 cle. Berkeley (1928) describes the organs as ven- 

 tral head muscles in Pandahts. 



CARAFACE ADDUCTOR MUSCLE 



Figures 33. 34. 37. 38 



The carapace adductor muscle originates on the 

 carapace just ventrad of the most ventral part of 

 the protocephalon attractor muscle (fig. 33). The 

 origin point of the carapace adductor is slightly 

 dorsal to the horizontally turned pleural plates, 

 above the gill pump, or scaphognathite. The mus- 

 cle runs directly mesad to insert on the postero- 

 dorsal midline of the endosternite (fig. 37). The 

 carapace adductor muscle functions as the major 

 position retainer of the endosternite. It may also 

 play a part in necessary distortions of the cara- 

 pace associated witli feeding, molting, and the like. 



The carapace adductor muscle appears in many 

 crustacean groups (Grobben 1917). Schmidt 

 (1915) describes it in Astacus as the musculus 

 dorsoventralis posterior and Berkeley (1928) has 

 adopted his terminology for the muscle in Panda- 

 lus danae. Grobben (1917) considers the cara- 

 pace adductor a useful phyletic character, because 

 of its frequent occurrence, and we are indebted to 

 this worker for the name. Grobben described the 



carapace adductor muscle (Schalenschliessermus- 

 kel) in species of Penaeus, Palaemon, Leander, 

 Pandalus, Galathea, and Athanas. 



DORSAL MUSCLES 



DORSAL THORACOABDOMINAL MUSCLES 

 Figures 34, 35 



Inserting on the anterodorsal teigum of the first 

 abdominal segment and running forward and 

 down to the laterotergal brachia of the thorax 

 are four pairs of long, slender muscles, the dorsal 

 thoracoabdominal muscles (figs. 34, 35). In dor- 

 sal view (fig. 35) tlie area of attachment may be 

 seen on the dorsal part of the first abdominal seg- 

 ment. The muscles divide around the heart and 

 hepatopancreas as they go to the lateral wall of 

 the thorax. Each muscle originates on its own 

 pleural arm, suggesting that the muscles each be- 

 long to specific thoracic segments. Upon contrac- 

 tion the dorsal thoracoabdominal muscles extend 

 the first abdominal segment witli respect to the 

 thorax, in opposition to the action of the anterior 

 thoracic muscles. 



The dorsal thoracoabdominal muscles are evi- 

 dently the same muscles as those designated as the 

 dorsal thoracoabdominal muscles in Pandalus and 

 Asfacu-:. although they appear to be relatively 

 larger in size in the caridean shrimp. 



LATERAL THORACOABDOMINAL MUSCLES 



Figures 34, 35 



From an area laterad of the heart and the dor- 

 sal thoracoabdominal muscles, the lateral thoraco- 

 abdominal muscles (figs. 34, 35) originate along 

 a diagonal line just above the dorsal edge of the 

 inner branchiostegal fold and run ventrally and 

 caudad to junctions with abdominal muscles. 

 Very likely the thoracoabdominal muscles are in 

 reality abdominal muscles. Unlike the anterior 

 thoracic muscles, the lateral thoracoabdominal 

 muscles are not segmented. At least four pairs of 

 these muscles covering a broad area laterally are 

 found in Penaeus. Functionally, the lateral thora- 

 coabdominal muscles are involved in the rlexinji of 

 the abdomen, reinforcing the action of the an- 

 terior thoracic muscles. Berkeley (1928) suggests 

 that contractions of the muscles on one side may 

 bend the abdomen laterally in Pandalus. and such 

 a movement may also take place in Penaeus. The 

 lateral thoracoabdominal muscles of Penaeus are 



