370 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 193 8 



tances had the highest average: (1) Total distance 338 km, time 29 

 days, or 12 km per day. (2) Total distance 257 km, time 43 days, or 

 8 km per day. (3) Total distance 368 km, time 38 days, or 10 km 

 per day. These large crabs in the spawning swarm consequently 

 travel downstream at the considerable rate of speed of 8 to 12 km 

 per day and thus can reach the breeding places in the brackish water 

 in 2 to 3 months even from the more remote regions. 



It is not surprising that the young animals cease their slow wander- 

 ing upstream in midsummer when the old animals commence their 

 rapid downstream migration, because the young animals are forced to 

 leave the channel in midstream when the two swarms meet. 



SHEDDING OF THE SHELL, GROWTH, AND LIFE SPAN 



The vertebrates, with their inner skeletons covered with a network 

 of living cells, grow evenly and inconspicuously, but arthropods, with 

 their dead outer armor, grow by leaps. The chitinous armor rein- 

 forced by deposits of lime must from time to time be thrown off and 

 renewed because it ages quickly, breaks and becomes useless. Besides, 

 growth is possible only by shedding the shell because the dead outer 

 shell cannot be enlarged through growth as do the bones of the 

 vertebrates. It must, therefore, from time to time be thrown off 

 and replaced with a new and larger one. Preparation for the shedding 

 of the armor is made in the forming of a new, thin, elastic shell under 

 the old one. The blood pressure now increases through the absorption 

 of water. The old armor bursts in the rear end between the carapace 

 and the abdomen, the crab glides out backward and expands at the 

 same time. The blood pressure produced to burst the old and expand 

 the new shell is so great that the crab after shedding can move on 

 its elastic legs without caving in. The new shell hardens through 

 absorption of calcareous deposits. This peculiar periodic mode of 

 growth with the shedding of the shell restricts the mitten crab to a 

 comparatively slow growth. In growth through sudden expansion, 

 limits are set to the elasticity, inasmuch as overexpansion would 

 tear many organs. According to investigations made by Dr. Schu- 

 bert, the length added by shedding is 24 percent in the smallest 

 animals but decreases as they increase in size and is only 11 percent 

 in the largest crabs, which are more than 70 mm long. The number of 

 sheddings per year are limited and evidently strictly regulated. The 

 gullet, the stomach, and the rectum, which reaches almost to the 

 stomach, are coated with chitin and are shed with the shell. They 

 must consequently be empty at the time of shedding. The shedding, 

 is, therefore, preceded by a period of fasting. The fasting continues 

 until the jaws harden. According to Dr. Schubert, the mitten crabs 

 shed 6 to 8 times during their first year, 4 to 5 times during their 



