154 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



(figs. 83, 84). In the posterior section of the gas- 

 tric mill, dorsal to the filter, is a structure here 

 designated as the dorsal (pyloric) gland (figs. 83; 

 85, E). The gland is mounted on the dorsal sur- 

 face of the gastric mill. Its function is unknown. 



The constrictor muscles of the gastric mill are 

 arranged rather like those of the esophagus, in 

 that the fibers are transverse to the long axis of 

 the gut and are attached to lateral folds. The 

 principal members of this group are the lateral 

 gastric mill constrictor muscles (fig. 83). These 

 attach between the long dorsolateral fold and the 

 deep lateral fold dividing the inward ventral 

 fold of the ventral channel. Contractions of the 

 lateral constrictors reduce the diameter of the gut 

 lumen and apply the lateral denticles to the gas- 

 tric mill contents. Ventral gastric mill constric- 

 tor muscles (fig. 83) bring the setose margins of 

 the ventral channel together. A third sheet of 

 fibers, the ventral longitudinal muscle (fig. 83) 

 runs between the lesser curvature of the gastric 

 mill and the anterior end of the filter. Their 

 function is not clear. 



In general, the constrictor muscle systems is 

 opposed by some 10 pairs of dilatator muscles. 

 Just dorsad of the esophagus are two small lat- 

 eral gastric mill dilatator muscles. Anterior to 

 these is an anteroventral dilatator muscle, in- 

 serted hard by its mate on the other side. Next 

 above is inserted the anterodorsal dilatator 

 muscle, also close to its counterpart at the mid- 

 line of the mill. On the anterodorsal surface of 

 the gastric mill, caudad of the anterodorsal dila- 

 tators, is the mesodorsal dilatator muscle, insert- 

 ing on the midline. Posteriorly, behind the curv- 

 ature of the gastric mill, is inserted a group of 

 perhaps three posteromedial dilatatpr muscles. 

 Ventral to the gastric mill is found the long, thin 

 posteroventral dilatator muscle which inserts 

 with its counterpart into the ventral midline at 

 the ventral curvature of the gastric mill. And 

 lastly, a ventral filter dilatator muscle inserts into 

 the ventral midline in the region of the filter. 



Gross movements of the gastric mill, as well as 

 operation of the median tooth, are accomplished 

 by the large anterodorsal gastric mill muscle and 

 the posterodorsal gastric mill muscle, both of 

 which insert into the dorsal part of the mill, in the 

 region of the median tooth. They are paired mus- 

 cles. Some of the actions of these muscles may be 

 observed in the living animal during feeding. 



HEPATOPANCREAS 



Figure 8:5 



The main digestive gland of Penaeus appears 

 to be the hepatopancreas. This large gland is 

 situated in the posterior region of the thorax, an- 

 teroventrad of the heart. It surrounds the junc- 

 tion of the gastric mill with the midgut. In con- 

 sistency, the hepatopancreas is a mass of closely 

 packed secretory tubules whose products ap- 

 parently are poured into the ducts of the hepato- 

 pancreas at the posterior end of the gastric mill 

 filter. 



B. Midgut 



The extent of the midgut, or mesenteron, in 

 Crustacea is evidently variable (Caiman 1909). 

 That part of the alimentary canal not sclerotized 

 in Penaevs runs from some point near the poste- 

 rior portion of the heptopancreas to the hindgut 

 gland in, the reader will recall, the anterior end 

 of the sixth abdominal segment. The midgut is 

 a simple, straight tube throughout its length. Its 

 association with the dorsal abdominal artery is 

 intimate. Very many small vessels connect the 

 two all the way from the heart to the hindgut 

 gland. Transverse sections of the midgut at vari- 

 ous points show that the gut lumen is lined with 

 low columnar epithelium resting on a basement 

 membrane. The outer layers appears to be con- 

 nective tissue in which muscle fibers are 

 distributed. 



C. Hindgut 



The proctodaeum, or hindgut, begins at the 

 hindgut gland and includes the rectum and anus. 

 Specimens cleared in strong alkali retain the cuti- 

 cular lining of this region of the gut. The hind- 

 gut (rectal) gland (figs. 61, 02, 74) projects dorsad 

 from the rectum. The gland is composed of 

 tubules lined with large secretory cells. The 

 tubules are blind dorsally and open into the dor- 

 sal surface of the rectum. The function of the 

 gland is unknown. The rectum (figs. 61, 74, 83), 

 too, is lined with secretory epithelium reflected 

 into deep folds. The gland cells of the hindgut 

 gland and rectum may play a part in osmotic 

 balance. The anus (fig. 82) consists of a bulb of 

 cuticle at the caudal end of the rectum and a large 

 tube passing ventrad to the anal opening. As 

 has been shown, the opening is controlled by 

 muscles. 



