26 Introduction to Animal Morphology. 



individual may be aplastide, an organ, an antimere, a 

 metamere, a persona, or a colony, and hence we speak of 

 individuals of the ist, 2nd, 3rd, 4th, 5th, or 6th orders. 

 Individuals of the 3rd to 6th degrees consist of organs 

 the study of whose mode of grouping is the science of 

 Tectology. Organs may be classified, according to 

 their functions, into : i. Assimilative; 2. Nutritive; 

 3. Depuratory ; 4. Relational ; and 5. Propagative. 

 In plastides the protoplasm fulfils all these functions. 

 With increasing complexity comes a division of phy- 

 siological labour and a localization of function, which 

 is termed differentiation, and seems due to the segre- 

 gation of some of the compound radicles which make 

 up protoplasm, myosin separating in one group of 

 cells, forming muscle, protagon, and myelin, in others 

 forming nerve, &c. 



Relational organs appear first to be differentiated 

 in the form of an integument ; indeed, the cell-wall in 

 plastides is of this nature. In groups of cells the outer 

 plastides harden into epithelium, and sometimes in 

 these cells, or even in the walls of a plastide, carbonate 

 of lime (Foraminifera) or silica (Radiolaria) becomes 

 deposited as a shell. Sometimes in plastides or groups 

 of plastides these substances are deposited as spicula 

 within the texture for purposes of support.* 



In higher forms a layer of connective tissue forms 

 under the epithelium constituting a dermis, from pro- 

 toplasm exuded on whose surface the epithelium is 

 renewed. In such cases the epithelium is moulded to 

 the secreting surface. 



* These deposits are formed in obedience to ordinary molecular force5, 

 as these mineral elements assume the same forms in viscid fluids, as proved 

 by the researches of Max Schultze, Rain,-\\ Harting, &c. 



