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COMPARATIVE MORPHOLOGY OF 

 VERTEBRATES 



THE INTEGUMENT 



The integument is the covering of the body, the term including 

 the skin (cutis) and all structures derived from it. From its position 

 it is a protective coat. It comes into relation with the external world 

 and is modified in various ways, becoming hardened to ward against 

 mechanical injury, developing sensory structures to give information 

 of untoward conditions and being impervious so as to prevent loss of 

 the body fluids, or the entrance of others from without. Naturally 

 the habitat, aquatic or terrestrial, has great influence in the character 

 of the modifications. 



In all vertebrates the integument consists of two layers, an outer 

 epidermis which consists of the ectoderm after the separation of the 

 nervous system, and a deeper layer, the cerium (derma) of mesen- 

 chyme, largely derived from the somatic wall of the myotomes, into 

 which other structures (nerves, blood-vessels, etc.) extend. 



In the epidermis, again, two layers are always present. At the 

 base, next to the corium is the Malpighian layer (stratum germina- 

 tivum), the cells of which are nourished by the fluids of the corium. 

 Hence they can grow and divide, the new cells thus formed gradually 

 passing to the outside where they form the second layer, the stratum 

 comeimi, the outer cells of which are usually worn away as fast as 

 new ones are added from below. Occasionally these outer cells come 

 off in large sheets, as when a salamander or a snake sloughs its 

 'skin.* In the development of the epidermis of the terrestrial 

 vertebrates the first layer of cells budded from the Malpighain stra- 

 tum form a continuous sheet which is later shed as a whole. This is 

 the periderm (fig. 20), the older name of epitrichiimi being inappro- 

 priate, since the layer is found in reptiles and birds where no hair 

 occurs. 



The Malpighian layer alone is concerned in the formation of the 



glands connected with the skin, and the corresponding part of the 



ectoderm contributes to the sensory structures like the nose and ear. 



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