CHAPTER 4 



THE INTEGUMENTARY SYSTEM 

 EVOLUTION OF THE INTEGUMENT 



Since all life involves continual adjustment of processes within the 

 organism to conditions outside, the skin and its appendages which mediate 

 this relation are highly important organs. 



Even among the protozoa, an external semipermeable membrane 

 separates the living protoplasm from the surrounding medium. Most 

 protozoa have in addition an outer differentiated layer of clearer cytoplasm, 

 the ectosarc, analogous in function to the skin of the higher animals 

 though without genetic relation. 



A true multicellular skin appears first in sponges and coelenterates, 

 the ectodermal layer of hydra being a familiar example. 



Even in so simple a skin as this, there is some differentiation among 

 cells. Most are epithelial covering cells, each commonly prolonged at its 

 base into a contractile thread. But among these are gland cells, which 

 by their different secretions in different coelenterates indicate a wide 

 difference in metabolic processes. The secretion of lime salts by the skin 

 of coelenterates may be regarded as the beginnings of the exoskeleton of 

 many higher invertebrates. 



Most invertebrates retain essentially unaltered the simple epithelial 

 ectoderm of coelenterates. Some have a ciliated epidermis which aids 

 locomotion. Many secrete an external cuticula, in which lime may or 

 may not be present. 



The evolution of a simple epithelium into a stratified epidermis, such 

 as occurs in vertebrates, results, presumably, from a change in the direction 

 of cleavage planes during cell multiplication. So long as cell walls form 

 perpendicular to the surface, a simple epithelium results. When, however, 

 cleavage planes form parallel to the surface, the membrane becomes 

 stratified. The outer layers of cells serve to protect the lower layer where 

 growth and cell multiplication take place. In animals exposed to dry 

 air, an outer layer of dead cells is obviously adaptive. Yet the beginnings 

 of a protective outer layer appear in the exoskeletons of water-dwelling 

 invertebrates. Among invertebrates appears also, though exceptionally, 

 a connective tissue layer or corium beneath the epidermis. 



The lowest chordates (balanoglossus, ciona, amphioxus), have both an 

 outer epidermis and an inner corium; but the epidermis is only a single 



164 



