observed (in a petri dish) under a dissecting micro- 

 scope, was largely by the pectoral fins which 

 "vibrated" seemingly with the rapidity of the 

 wings of a bee in flight. The sinuous body move- 

 ment was not abandoned, but the frequency had 

 slowed enough that it could be followed easily. 



When food was added to the aquarium, the 

 young shad darted to and fro as though pursuing 

 some of the small particles. Although the di- 

 gestive tract contained food during the fifth or 

 sixth day, only green algae were recognized 

 through the thin gut wall. 



The young shad congregated on the lighted 

 side of the aquarium. If all sides were lighted, 

 they remained mostly near the source of the 

 running water, facing into the current. 



The first few days after hatching, the young 

 gizzard shad subsists on its yolk. Increase in 

 length of the 3.5 mm. newly hatched larva at 

 this time results largely from the straightening of 

 the cephalic flexures, after which the larva attains 

 a length slightly greater than 5 mm. About the 



5th day it begins to feed and by the 10th day has 

 attained a length of slightly more than 6 mm. 

 (in the laboratory). At this time it has a filiform 

 shape, which it retains until it has reached a 

 length of 20-30 mm. (fish from the lake — age 

 unknown). Upon reaching this length, the shad 

 begins to increase in depth. The filiform shape 

 gives way to the slab-sided, deep-bodied adult 

 form. Once this shape is acquired the form 

 changes little and the shad begins to grow rapidly. 



DIGESTIVE TRACT 



Development of the Digestive Tract 



The gut of a newly hatched gizzard shad is a 

 tube which conforms to the body curvature. In 

 the 10-day-old larva the gut is a simple, nearly 

 straight tube. Among the older shad the gut of an 

 18-mm. fish (age unknown) had already developed 

 two flexures (fig. 14). The 19-mm. shad had 

 begun the third and fourth flexures, and the 22.5- 

 mm. fry had already completed them. The portion 



ii - -  --*— * 



18 MM. SHAD (SL) 



19 MM. SHAD (SL) 



33 MM. SHAD (SL) 



22.5 MM. SHAD (SL) 



27.5 MM. SHAD (SL) 



51 MM. SHAD (SL) 

 Figure 14. — Development of the shad gut (camera lucida drawings). 



418 



U.S. FISH AND WILDLIFE SERVICE 



