3l>8 APPLIED BIOLOGY 



we owe all that has placed man so far above ordinary animal 

 life. But such high development of the nervous aiul mus- 

 cular organs, whose activities make human life worth living, 

 would have been impossible without parallel increase in the 

 complexity of those organs (digestive, respiratory, circula- 

 tory, and excretory) upon which the muscular and nervous 

 organs oVpoiul. 



312. Molting of Crayfish. It is evident that an animal 

 inclosed in a hard external skeleton like that of the crayfish 

 cannot grow rapidly while surrounded by such a coat-of-mail. 

 This difficulty is overcome by periodical shedding of the 

 skeleton, followed by rapid increase in size for a short time 

 while the skin remains soft and extensible. The shedding 

 or molting occurs several times in the first year of a cray- 

 fish's life when it grows rapidly, and less frequently there- 

 after. The process of molting is in essentials as follows: 

 The membrane on the dorsal side at the joint between the 

 carapace andlihe skeleton of the abdomen breaks, and through 

 the opening thus made in the skeleton the animal works 

 out the head-thorax and its appendages from the carapace 

 and withdraws the abdomen from its skeleton. As the ani- 

 mal emerges from its old skeleton, it is seen to be covered with 

 a soft new skeleton; and within a short time it expands 

 enormously, largely because of the absorption of water. 

 Gradually lime salts become deposited and cause the harden- 

 ing of the new skeleton. For a long time new internal tissues 

 may be formed, this new growth displacing some of the water 

 which caused the animal to swell suddenly when released 

 from its old skeleton. It is evident that the sudden absorp- 

 tion of water when molting makes the skeleton large enough 

 to allow for growth (forming new cells) for a long time, per- 

 haps for a year. Owing to the very rapid growth in the first. 

 year, a lobster five inches long has probably molted twenty 

 times, while a ten-inch lobster has molted twenty-five timrs. 



Such a molting process occurs in all arthropods, but the 



