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 in- 

 formation of untoward conditions and being impervious so as to prevent 

 loss of the body fluids or the entrance of others from without. Natur- 

 ally 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 corium (derma) of mesenchyme, 

 derived from the somatic wall of the myotomes, into which other struc- 

 tures (nerves, blood-vessels, etc.) extend. Strictly speaking the 

 bony scales of fishes are integumental, but on account of their close 

 relations to the skeleton they are best treated in that connexion. 



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

 next to the corium is theMalpighian layer (stratum germinativum) , 

 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 corneum, 

 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 Malpighian stratum form a continuous 

 sheet which is later shed as a whole. This is the periderm (fig. 17), 

 the older name of epitrichium being inappropriate, since the layer is 

 found in reptiles and birds where no hair occurs. 



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