between the first and second flexures (later to 

 become the gizzard) had enlarged. By the 

 27.5-mm. stage, the caeca had begun to develop 

 from the duodenum (between the second and third 

 flexures). As the gut continues to grow, the 

 flexures move until they reach the limit of the 

 abdominal cavity. Since the gut grows much 

 faster in length than the abdominal cavity, 

 secondary flexures are produced between the 

 primary ones. Flexures provide landmarks by 

 means of which one can determine relative rates of 

 growth of the different portions of the gut. The 

 caudal portion grows more rapidly and continues to 

 grow over a longer period than any other section. 

 No secondary flexures develop anterior to the 

 third primary one. 



Although the course of the gut of a young giz- 

 zard shad is complicated, it is relatively simple in 

 comparison with that of the adult of, say, 250 mm., 

 in which the convolutions are so numerous and 

 complex that they defy tracing on a two-dimen- 

 sional illustration. Indeed, the gut which amounted 

 to one-half of the total length of the day-old 

 fish becomes, in the adult, three times the length 

 of the fish and is packed in an abdominal cavity, 

 one-third of the fish's length. 



Forbes (1914) stated that the larva of the gizzard 

 shad has teeth. Although I have examined many 

 young, from newly hatched to longer than 20 mm. 

 (when the adult shape begins to take form), I have 

 not observed teeth. 



Digestive Tract in the Adult 



The hundreds of long, thin gill rakers of the 

 gizzard shad are admirably adapted for removing 

 particulate matter from the water. This ma- 

 terial, especially the filamentous algae and the 

 small crustaceans, apparently is accumulated in 

 the pharyngeal pockets, a paired muscular organ 

 mentioned by Forbes (1888) and described by 

 Lagler and Kraatz (1944). The thick muscular 

 wall and the direct connection with the esophagus 

 suggest that these pharyngeal pouches force their 

 contents into the esophagus. The sphincter 

 muscles in the forepart of the pneumatic duct 

 prevent passage of food into the air bladder. 



The muscular esophagus and the gizzard have 

 been described by Wier and Churchill (1945). 

 I can add only that the esophagus possesses 

 longitudinal folds. 



Numerous caeca arise in groups on the lateral 



GIZZARD SHAD IN WESTERN LAKE ERIE 



surface of the duodenum. Each group has a 

 common orifice through which the lumina of its 

 caeca communicate with the lumen of the duo- 

 denum. These orifices are arranged in two parallel 

 longitudinal rows. In a 200-mm. shad the lengths 

 of the caeca range from about 5 mm. for those at 

 the anterior end of the row to about 2 mm. at the 

 posterior end. Counts were not made, but an 

 individual appears to possess several hundred 

 caeca. The caeca have internal longitudinal folds. 

 Their lumina are so small that only the smaller 

 unicellular organisms can enter. Although the 

 caeca have the histological appearance of absorp- 

 tive devices rather than secretory organs, it is only 

 when the duodenum is turgid with food that any 

 material is to be found here. 



Wier and Churchill (1945) described a pancreas 

 separate from the liver for the gizzard shad. I have 

 been unable to identify one although I have 

 searched for it repeatedly in many sizes of fish. 

 These authors described the liver as composed of 

 several lobes. I find that it has no definite form 

 in the adult fish but rather seems to spread in close 

 proximity to the gut. The liver invades inter- 

 caecal spaces as well as those between neighboring 

 portions of the gut, and completely covers the 

 anterior, dorsal, ventral, and left-lateral surfaces 

 of the gizzard. It is a diffuse organ and intermixed, 

 I believe, with the pancreas. 



The small intestine possesses no villi, contrary 

 to the statement of Forbes and Richardson 

 (1908). It does contain four of the large, con- 

 spicuous, longitudinal folds described by Wier and 

 Churchill (1945). Attached transversely between 

 the folds and along the inner circumference of the 

 gut are smaller folds or lamellae which have the 

 free edge directed toward the center of the gut 

 lumen (fig. 15). The free edge of each lamella 

 is directed posteriorly at a slight angle. Although 

 the columnar cell covering of these transverse 

 lamellae is unquestionably absorptive and these 

 structures greatly increase the absorptive surface 

 of the gut, they may also function in the manner 

 suggested to me by Milton B. Trautman. He 

 suggested that during peristalsis the longitudinal 

 ridges may lengthen and shorten, thereby causing 

 the lamellae to move back and forth in venetian- 

 blind fashion and, hence, to aid in forcing food 

 along the tract. 



This lamellar arrangement within the gut bears 

 some resemblance to the situation described by 



419 



