SPECIAL PART. 

 A. INTEGUMENT. 



THE skin consists of a superficial ectodermal layer, and a 

 deeper mesodermal layer. The former is called the epiderm 

 (scarf-skin) and the latter the derm (corium, cutis). 



In the epiderm, which consists of cells only, two parts may in 

 general be distinguished : an external layer, composed of flattened 

 and hardened cells (stratum corneum, ~kwny layer), and a deeper, of 

 more columnar, formative cells (stratum Malpighii s. germativum, 

 mucous layer). The latter serves for the regeneration of the 

 horny layer, the superficial part of which is continually scaling 

 oft , as well as for the formation of such horny structures as hairs, 

 bristles, nails, claws, and hoofs, and of the integumentary glands. 

 The peripheral sensory end-organs also arise by a differentiation 

 of this layer of cells. 



The derm, which is usually thicker and tougher than the 

 epiderm, is made up principally of connective tissue and smooth 

 muscular fibres : it is usually not sharply marked off from the 

 subcutaneous connective tissue, which commonly encloses more or 

 less fat. Externally, the derm may give rise to numerous papillae 

 projecting into the epiderm, especially in higher forms. Apart 

 from the horny and glandular structures extending into it from 

 the epiderm, the derm encloses vessels, nerves, and often l>ony 

 structures also. 



Pigment cells (chromatophores) and free pigment occur in both 

 layers of the skin : they correspond to modified connective tissue 

 cells, and in them a temporary shifting of the contained pigment 

 may occur, this process being under the control of the nervous 

 system. 



In Amphioxus, the epiderm differs from that of all the Craniata 

 in the fact that it consists of a single layer of cells : its surface is 

 covered with cilia in the larval (gastrula) stage, and this must 

 undoubtedly be considered as an inheritance from Invertebrate 

 ancestors. 



