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 the Malpighian 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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