ECDYSIS 345 



coiling the cone into a spiral by more rapid growth on one 

 side. This device is also very generally employed wherever 

 the cone is in use (many Protozoa, some Worms, Gastropods, 

 Cephalopods, especially extinct forms). 



731. One of the most distinctive characters of the entire 

 phylum of Arthropods is the way the problem of growth me- 

 chanics is solved. The Arthropod is entirely enclosed in a 

 sheathing of chitin, a substance which is very elastic but has very 

 little power of stretching. In fact, the animal cannot grow while 

 encased in this armor. Consequently, the armor is removed peri- 

 odically and then a period of rapid expansion ensues until the 

 new shell has hardened again. Among the Crustaceae the casting 

 of the shell (ecdysis) occurs frequently during the early periods 

 of development (lobster 7-8 times in first year), later the moult- 

 ing periods are less frequent (once per year, crab, lobster). 

 During the soft-shell periods the animal remains concealed in 

 some cranny, because it is then extremely helpless. Not only 

 is it unprotected by a shell, but its " claws, " at other times so 

 formidable, are now useless. Nevertheless, the Crustaceae as a 

 class have been very successful, and we must conclude that the 

 disadvantages of the period of ecdysis are more than compen- 

 sated by the advantages of the chitinous armor. 



732. The more primitive Insects follow in general the Crus- 

 taceae in regard to the management of this armor, but most 

 orders of Insects have adopted a different and probably a better 

 plan. Diptera, Coleoptera, Hymenoptera and Lepidoptera, the 

 most numerous orders, develop by metamorphosis. Their 

 larvae are soft-skinned and are in various ways enabled to dis- 

 pense with the armor. During the pupal stage they are usually 

 concealed in the earth or elsewhere, and after they emerge as 

 completely armored insects, they no longer grow. Their growth 

 is completed and no ecdysis is needed. Ecdysis occurs at the 

 period of pupation, and again at the emergence of the imago, 

 and at this time the insect is often concealed. 



