CHAPTER LXVI 

 PROTECTIVE ARMORS OF ORGANISMS 



398. Walls and shields. The simple cell wall that we find 

 in the one-celled plants, and the cell membrane found in many 

 one-celled animals, may be considered to serve as protection 

 against mechanical injury to the protoplasm. At the same time 

 they permit the osmotic transfer of income and excretion. 



Fig. i6o. The horseshoe crab 



This animal is protected by an external skeleton, or armor, of chitiu secreted by 



the skin cells 



In plants and animals made up of many cells we generally 

 find that the external layer of cells is either modified into 

 a protective layer or supplemented by various protective struc- 

 turesv The outer cell walls of skin cells in plant structures 

 are usually thicker than the inner walls and much thicker 

 than the walls of inside cells. The skin cells of leaves usually 

 have a secretion of a fatlike substance on the outer surface 

 (see /, Fig. 5), called cjitin, which prevents evaporation from 

 within as well as water-logging from without. It adds also 

 to the protection against mechanical injury. 



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