TEGUMENTARY ORGANS 371 



line between liber and wood of plant, between bone and periosteum of 

 animal ; b, b\ cork and epidermic layers of plant ; cellular epidermis 

 and scale of animal, fish, e.g. ; c, mesophlceum, enderon (derma) ; d, liber, 

 periosteum ; e, e , wood and pith, bone and cartilage ; x, axis ; y, 

 surface. 



The consideration of vegetable structures will aid us even further in 

 understanding the manner in which the different varieties of in- 

 tegumentary organs, with which we shall meet, are formed. For it is 

 well known that the outer covering of a plant may ultimately be 

 constituted in one of three ways. i. The original cellular ecderon 

 may persist unchanged. 2. The " epiderm '' persisting, a laminated, 

 but otherwise structureless " ciitiaila^' may be developed upon its 

 outer surface, attaining sometimes a very considerable thickness. 

 3. The original epidermis is cast off, its place 

 being taken by the development of a new layer 2/ T 

 of different, usually suberous constitution be- i< 



neath it, which then goes on growing endo- L 



genously, and constitutes the permanent in- 

 tegumentary surface. Now, we find a precise „ 

 parallel for all these conditions in animals. In "'- — 

 the soft integument of most Mollusca and Ver- f 

 tebrata the first condition obtains, the general 



surface of the integument being constituted by x 



the cellular " epidermis." Fig. 304. 



In the Annulosa, on the other hand, the 

 integument has certainly, in many cases, and I think probably in 

 the great majority, the character of a vegetable cuticle, consisting as 

 it does of layers developed from the outer surface of the cellular 

 ecderon. In this way also I believe that all molluscan shells are 

 formed. 



Lastly, the fish-scale produced altogether beneath the cellular 

 ecderon or epidermis, but growing endogenously after the manner of 

 a true ecderonic structure, appears to be precisely analogous to the 

 corky periderma of the plant ; and as the latter, though it is not the 

 original epidermis, takes its place and grows in the same way, so in 

 the fish the scale, which is assuredly not a calcification of the 

 cellular ecderon, yet represents it both in position and in mode of 

 growth. 



§ 2 Morphology of the integuments. — In the embryonic state of all 

 animals, and in the adult condition of many of the lower forms, the 

 integument, constituted as above defined, forms a continuous invest- 

 ment over the surface of the body without any important processes or 



B B 2 



