WHITE SHRIMP FROM THE GULF OF MEXICO 



65 



parently similar muscle in the palp of Astacus is 

 called by Schmidt (1915) a palp flexor also. 

 Berkeley (1928) describes both a palp flexor and a 

 palp extensor in the body of the mandible of 

 Pandalus. 



MANDIBULAR PALP ADDUCTOR MUSCLES 

 Figure 29 



The proximal, and larger, mandibular palp ad- 

 ductor muscle originates on the posterior wall of 

 the gnathal lobe of the mandible and inserts on 

 the medial margin of the basal palp foramen. 

 The smaller palp adductor originates dorsad of 

 the insertion of the larger palp adductor on the 

 medial margin of the gnathal lobe foramen. Con- 

 tractions of these muscles turn the palp segments 

 toward the midline. The proximal mandibular 

 palp adductor muscle is probably homologous 

 with the museums flexor a mandibulae of Astacus 

 and either the palp flexor or extensor of Pandalus. 



2. PARAGNATHA 



The paragnatha (figs. 29, 39) are two rounded 

 lobes suspended from small foramina in the ven- 

 tral skeleton immediately posterior to the gnathal 

 lobes of the mandibles. Their cuticle is very thin, 

 except for a slightly thickened ridge along the 

 posterior surface of each paragnath. No intrinsic 

 muscles are found in the body of the paragnath in 

 Penaeies; however, a small muscle, the para- 

 gnathal muscle (fig. 39), inserts on the lateral 

 margin of the paragnath. The paragnathal mus- 

 cle moves the paragnath laterally and anteriorly 

 against the gnathal lobe of the mandible. 



Classically, the paragnatha have been inter- 

 preted as a secondary development of the mandib- 

 ular segment, on grounds of their embryonic de- 

 velopment and because of their innervation by 

 mandibular nerves. Also, the view that they be- 

 long to the maxillae has been expressed. The idea 

 that the paragnatha are reduced true appendages 

 has in general been discounted, despite the pres- 

 ence of movable terminal lobes in the paragnatha 

 of Tanaidacea (Crustacea). Snodgrass (1935) 

 mentions the similarity of the insectan super- 

 linguae to the crustacean paragnatha. This 

 worker feels, apparently, that there is no evidence 

 that the paragnatha are appendicular or that the 

 superlinguae and paragnatha are homologous 

 structures. He does indicated (Snodgrass 1952) 

 in a study of Cambarus, that the paragnatha are 



B 



Figure 39. — Paragnatha. A. Anterior view. B. Posterior 

 view. 



mandibular entities. In a comparative study of 

 arthropod nerves, Henry (1948a) assigns the 

 paragnatha to the mandibular segment by virtue 

 of their innervations. She invariably places the 

 paragnathal nerves in a position posterior to those 

 of the mandibles. 



Recently, Chaudonneret (1955, 1956), in a de- 

 tailed study of the gnathal nerves of Orconectes 

 limosm (Eafinesque) ( = Cambarus affinis Say), 

 takes issue with Henry (1948a) and advances the 

 idea that the paragnathal nerves are in fact an- 

 terior to those of the mandibles and entirely dis- 

 tinct from the mandibular nerves. On other 

 grounds, furthermore, this worker suggests that 

 the paragnathal foramina are anterior to the man- 

 dibular foramina and holds that their relative 

 positions with respect to the mouth are constant 

 in the Malacostraca. In the opinion of Chau- 

 donneret (1956), the facts make difficult the inter- 

 pretation of the paragnatha as either epithelial 

 lobes or parts of the mandibles. Instead, this in- 

 vestigator thinks that the paragnatha may belong 

 to a reduced premandibular, paragnathal segment 

 homologous to the insectan superlingual segment. 



If the paragnatha are indeed premandibular 

 and homologous to the superlinguae, an interpre- 

 tation which Snodgrass (1935) seems to consider 

 possible, certain aspects of the morphology of the 

 insect hypopharynx will need review. Careful 

 study of the paragnathal nerves in Penaeus tends 

 to support the view of Chaudonneret (1956) that 

 this nerve is slightly anterior to the mandibular 

 nerve. However, the gross anatomy will have to 



