WHITE SHRIMP FROM THE GULF OF MEXICO 





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63 



dible articulates with the mandibular segment at 

 a single point laterally, an arrangement which 

 permits the varied movements of the organ seen 

 at this stage of its evolution. 



The monocondylic mandible is found in all the 

 mandibulate arthropods except some higher Crus- 

 tacea and the pterygote insects (Snodgrass 1935). 

 In the latter forms, a secondary condyle on the 

 epistome is added mesad of the primary lateral 

 condyle. Mandibular movements now become re- 

 stricted to those about the dicondylic axis. With 

 simplicity, furthermore, comes strength. Snod- 

 grass (1935, 1951b) has described the evolutional 

 simplification of mandibular musculature which 

 follows in groups developing the dicondylic 

 mandible. 



A comparison of the mandibular hinges and 

 musculature of the white shrimp with the account 

 of Snodgrass indicates that Penaeus represents a 

 transitional form; for, although the shrimp man- 

 dible is dicondylic, its musculature is strongly 

 reminiscent of the monocondylic musculature in 

 apterygote insects, myriapods, and lower Crus- 

 tacea. 



The endoskeleton of the mandibles, mentioned 

 above, is a thick tendon situated between the 

 mandibles and upon which the heavy ventral ad- 

 ductor and abductor muscles insert (figs. 37, 38). 

 The substance of the endoskeleton is extremely 

 tough connective tissue, not sclerotized. Such 

 cuticular components of the structure as may 

 exist are not hard. The mandibular endoskeleton 

 of Crustacea Malacostraca has been variously in- 

 terpreted. In forms having a well-developed 

 endoskeletal system in the thorax, the mandibular 

 element is considered to be a part of that system. 



In Astacus, Schmidt (1915) refers to the struc- 

 ture as the endoskeleton, specifically, the head 

 apodeme. Snodgrass (1935), referring to a simi- 

 lar structure in the mandibles of a diplopod, uses 

 the term median ligament. Whether this worker 

 attributes the median ligament to the endoskele- 

 ton is not clear. Berkeley (1928) designates the 

 same material in Pandalus as the anterior fascia. 

 She evidently considers it to be endoskeletal in 

 nature, and the fusion product of the mandibular 

 and first maxillary segments. Grobben (1917) 

 apparently believes the mandibular tendon to be 

 endoskeletal, naming it the median transverse 

 mandibular tendon. Snodgrass (1952) refers to 

 an intergnathal ligament in Anaspides tasmaniae 

 Thomson and an endosternum in Cambai'us lon- 



gulus Girard, in speaking of the structure. In the 

 course of a histological study of muscular and 

 skeletal elements in various Crustacea, Debaisieux 

 (1954) emphasizes that the mandibular endo- 

 skeleton is an endosternite, fully homologous in 

 his opinion with that of Arachnida. 



No further evidence is offered in the present 

 study for the homology of the endosternite of 

 Arachnida with the mandibular endoskeleton of 

 Crustacea. However, from the work of Debaisieux 

 (1954) on the structure in Crustacea, we can be 

 fairly certain that the mandibular endoskeleton of 

 all crustaceans mentioned above are homologous. 

 In response to the plethora of names for the man- 

 dibular endoskeleton, the present writer sees no 

 objection to the terms "endosternite" or "endo- 

 sternum" given by Debaisieux (1954) and Snod- 

 grass (1952). These names are used here as 

 equivalent to one another and to the expression, 

 "mandibular endoskeleton." Whether an endo- 

 skeleton and an endosternite are morphologically 

 equal is not made clear in the literature. 



As a final word on the composition of the endo- 

 skeleton, it will be recalled that Schmidt (1915) 

 describes two small medial muscles between the 

 mesophragms of the endosternite, the endo- 

 phragmal compressor muscles. Grobben (1917) 

 denies the existence of muscle fibers in this mate- 

 rial in the crawfish and establishes that the area 

 is composed of connective tissue. 



MUSCLE ELEMENTS 



MANDIBULAR ABDUCTOR MUSCLES 

 Figures 33, 37, 38 



At least three mandibular abductor muscles 

 (figs. 33, 37, 38) are found in Penae-us. The 

 smallest is the most anterior. This muscle arises 

 in connective tissue in the anterior region of the 

 gnathothorax, lateral to the esophagus and dorsal 

 to the circumesophageal connective, and runs pos- 

 teriorly and ventrad to insert on the anteromedial 

 part of the mandibular endoskeleton. Contrac- 

 tions of these slender muscles aid in opening the 

 mandibles. No counterpart of the muscle is de- 

 scribed for the other crustaceans to which ref- 

 erence has been made. 



The large and important mandibular abductors 

 originate in connective tissue on the laterotergal 

 plates (fig. 33) and run posteromedially to inser- 

 tion areas on the mandibular endoskeleton (figs. 

 37, 38). Their contractions serve to open the 



