GENERAL ORGANOLOGY 91 



so insignificant that they cannot be seen at all by the naked eye, and scarcely 

 with the microscope. 



Nevertheless the two terms, animal and vegetative, must be retained. For 

 although motion and sensation occur in the vegetable Icingdom, still they reach 

 no high development; they become more and more inconspicuous the higher 

 the plants; in the animal kingdom, on the contrary, they are unfolded in extra- 

 ordinary perfection and lie at the basis of its most characteristic features. 



SKIN AND SKELETON. 



With a few exceptions (cestodes, trematodes, nematodes) which are 

 not fully explained, the outer surface of the Metazoan body is covered 

 with a typical epithelium, commonly called epidermis and occasionally, 

 from its origin, ectoderm as well. In invertebrates and in AmpJiioxiis 

 it is one layered, but in all true vertebrates it is stratified. To this im- 

 portant part of the integument there is usually added the mesodermal 

 part of the skin which is especially developed in vertebrates (coriutn or 

 aitis, derma) and is derived from the connective tissue. The skin is 

 frequently concerned in skeleton formation, as when in Coelenterates, 

 molluscs and arthropods it secretes on the outer surface of the epidermis 

 a cuticular armor, frequently hardened by lime. On the other hand there 

 may be a calcification (echinoderms) or an ossification of the corium 

 (scales of fishes, bony plates of reptiles and mammals). This dermal 

 skeleton is in strong contrast to the axial skeleton of vertebrates which 

 consists of cartilage and ossifications in the interior of the body. 



Vegetative Organs. 



A. Organs of Assimilation. 



Assimilation Defined. — If the term assimilation be used in its 

 widest sense, it includes all the contrivances in the animal body which 

 render growth possible during the period of progressive development, 

 and, during mature life, compensate for the loss of energy connected with 

 each functional act, in order to preserve to the body its functional powers. 

 With each functional act organic compounds are oxidized. Compounds 

 which are especially rich in carbon and hydrogen (as well as some nitrogen 

 and sulphur) and are poor in oxygen are changed by oxidation into carbon 

 dioxide, water, and various nitrogenous products, like urea, uric acid, etc. 

 A compensation takes place, for not only is the useless substance removed, 

 but also compounds of oxygen and materials rich in carbon are furnished 

 to the tissues to replace the material oxidized. An apparent exception 

 is furnished by the 'anaerobic' organisms which live, move and per- 

 form work without oxygen. Aside from some bacteria, which are outside 



