THE ORGANS. 37 



stages in the differentiation of the animal organism to later, and there- 

 fore higher stages, and which, following the law of the division of 

 labour, give rise to series of new organs. We do not yet know enough 

 of the details of the organological differentiation of the germinal 

 layers to be able to give the history of every organ. However, the 

 facts which are clearly established with regard to, at any rate, some 

 divisions of the Animal Kingdom enable us to follow out the first 

 steps in the process of differentiation. The organs which put the 

 organism into relation with the outer world, such as organs of 

 defence, of support, and of sensation, are principally derived from 

 the ectoderm (hence called the sensory layer), also those of move- 

 ment ; while the endoderm principally provides the organs for the 

 preservation of the individual and of the species (nutritive layer). 

 As the origin of the mesoderm, out of which important organs 

 are formed, is still very obscure, the relations of these organs to one 

 or other of the two primitive germinal layers must be left an open 

 question. 



The primitive character of the organism more or less disappears 

 as the rudiments of the body are formed out of the germinal layers, 

 and as fresh organs which render the organism more complicated 

 arise in it. Organs differentiated from the germ layers which act 

 the part of primitive organs are reckoned as secondary organs. 

 From these, tertiary organs are formed, and so on. The separate 

 organs differentiated out of a primitive organ remain connected 

 together, owing to the fact of these processes of separation being due 

 to the division of a function, and of the separate functions being 

 subordinated to the primary function, from the breaking up of which 

 they took their origin. Combinations of organs are therefore 

 formed, which are known as organic systems, on account of their 

 morphological and physiological connection. 



This connection does not always persist in the adult condition ; 

 and, in fact, organs primitively connected often become separated. 

 This obtains chiefly in those organs which serve several purposes, 

 for when the functions become independent the organs become so 

 too. But even in these cases ontogeny indicates what was the 

 primitive condition. 



Systems of Organs, 

 a) Integument. 



§ 30. 



The ectoderm, as the outermost layer of the body, forms the 

 simplest condition of the integument of animal organisms. In 

 the lowest organisms (Protista) there is either no integument at all, 

 the protoplasm which forms the body being protracted into ever- 

 changing processes (pseudopodia), or the integument is represented 



