150 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



IV. DIGESTIVE SYSTEM 



The digestive system is made up of those struc- 

 tures concerned with the enzymatic and mechani- 

 cal reduction of food particles, the absorption of 

 their products, and the evacuation of particulate 

 wastes. The digestive systems of Crustacea De- 

 capoda are all relatively uniform, being comprised 

 of a stomodaeum, or foregut, which includes the 

 esophagus and gastric mill, and probably the 

 hepatopancreas ; the midgut, or mesenteron, which 

 in Pendens appears to run from the gastric mill 

 to the hindgut gland on the anterior end of the 

 sixth abdominal segment ; and the hindgut, or 

 proctodaeum, which includes the rectal gland, 

 rectum, and anal canal. The alimentary struc- 

 tures will be treated according to these three re- 

 gions. The subdivisions of the gut are based 

 upon the presence of a cuticular lining of the fore- 

 and hindgut. That section not lined with cuticle 

 is supposed to be the midgut. This criterion is 

 here adopted for whatever it is worth. 



A. Foregut 

 ESOPHAGUS 



Figures 83 to 85 



The esophagus is a vertical tube which receives 

 food particles thrust into the mouth by the labrum 

 and post oral mouthparts. The tube is lined with 

 cuticle, yet is so constructed that extensive con- 

 traction and expansion is possible. A cross-sec- 

 tional view of the esophagus (fig. 85, .4) indicates 

 how these movements come about. The cuticular 

 lining is folded inwards between the corners. 

 Esophageal constrictor muscles (fig. 83), short 

 fibers running transversely, draw the corners to- 

 gether and thereby close the lumen of the tube. 

 Expansion of the lumen is accomplished by an- 

 terior and posterior esophageal dilatator muscles 

 (fig. 83) which are inserted into the inward folds. 

 Their contractions pull the folds outwards. The 

 esophagus is constricted at its junction with the 

 gastric mill preventing backflow of the food parti- 

 cles from the latter. 



GASTRIC MILL 



Figures 83 to 85 



The gastric mill is a bulbous, folded structure 

 occupying the dorsomedial region of the thorax. 

 As decapod gastric mill structure goes, that of 



Penaeus is relatively simple. Its surface is in- 

 vested with thin sheets of constrictor muscles. 

 Numerous dilatator muscles attach about the 

 outside of the structure. The gastric mill, like 

 the esophagus, is lined with cuticle, but here the 

 latter substance is reflected into deep folds and 

 toothlike structures (fig. 84) for the grinding of 

 food. Transverse sections through the mill (fig. 

 85) indicate that the theoretical tubular form of 

 the gastric mill is much modified. In general, 

 the walls have been thrown into three pairs of 

 folds. The uppermost fold (fig. 85, B), which 

 disappears caudally (figs. 85, C\, et seq.), serves 

 no other purpose than the expansion and contrac- 

 tion of the gastric mill in response to feeding and 

 digestion. 



The middle fold, in contrast, constitutes a mov- 

 able ridge or shoulder upon which a row of in- 

 creasingly large teeth, the lateral denticles (fig. 

 84; 85, B, C\, C 2 ) are borne. These apparently 

 play an important role in the reduction in size of 

 the gastric mill contents. The lower, or ventral, 

 fold (fig. 85) is separated from the dorsal folds by 

 an extensive lateral fold. The inner margin of 

 the ventral fold is setose. The setae tend to set 

 apart a ventral channel, or channels, running the 

 length of the gastric mill. This ventral channel 

 broadens posteriorly (fig. 85, D) and in the pos- 

 terior end of the gastric mill the lateral ends of the 

 channel bend ventrad (fig. 85, E) . Here, the chan- 

 nel enters a filter (fig. 84) which is made up of the 

 closely appressed, setose walls of the ventral chan- 

 nel. The setae are arranged in regular rows. Ac- 

 cording to Caiman (1909), the filter is very con- 

 stant in the penaeids. At its posterior end, the 

 filter has a pair of ducts through which the diges- 

 tive fluids of the hepatopancreas enter the gastric 

 mill (fig. 85, E). Presumably the filter prevents 

 the entry of particulate matter into the main col- 

 lecting ducts of the hepatopancreas. 



The principal device for chewing in the gastric 

 mill is the median tooth, situated in the midline 

 at the dorsalmost point of the mill (figs. 84; 85, C 2 , 

 D). The median tooth is a heavily sclerotized 

 structure, a proliferation of the gastric mill wall. 

 The tooth rests on articular rods which enable its 

 operating muscles to move it some distance into and 

 out of the lumen of the mill and with considerable 

 force. 



Posterior to the regions of the median tooth and 

 beyond the filter the terminal lappets of the gas- 

 tric mill project into the lumen of the midgut 



