MORPHOLOGY OF ANIMALS 



may give rise to various types of glands, such as sweat, 

 oil, wax, and milk glands, all of which are epidermal in 

 origin (Fig. 7). 



{b) Dermis. Beneath the surface epithelium, which is 

 always ectodermal in origin, there is in many animals a 

 fibrous or connective tissue layer known as the dermis or 

 corium. This layer is derived from mesenchyme and is 

 sometimes called "leather-skin," since leather comes from 

 it. It is especially well developed among echinoderms and 

 vertebrates, in both of which it may give rise to skeletal 

 spicules or plates, thus forming a dermal exoskeleton 

 (Fig. 10, ^/, sp). Among the vertebrates this is especially 

 well developed in fishes, the scales which cover the body 

 being of dermal origin ; in some cases these dermal scales 

 are covered by enamel which is derived from the epider- 

 mis. The same is also true of the teeth of vertebrates ; the 

 inner portion or dentine is of dermal origin, while the 

 enamel comes from the epidermis; teeth are in fact only 

 modified scales (Fig. 6, B and C). 



Skeletal System 



An internal skeleton, not the product of the integument, 

 is present in relatively few invertebrates, but is found in 

 all vertebrates and is always derived from mesenchyme. 

 Such a skeleton is found in sponges in the form of cal- 

 careous, silicious and horny spicules; in cnidarians and 

 ctenophores, as a supporting jelly; in many invertebrates, 

 as a system of connective-tissue cells and fibres; in cepha- 

 lopods and certain arthropods, as cartilages surrounding 

 the central nervous system. 



On the other hand the possession of a primitive axial 



C 25 •} 



