28 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENEEAL. 



which are placed one behind the other, and more or less completely 

 resemble each other in structure (Annelids, fig. 15). The successive 

 segments may in structure and function appear completely equiva- 

 lent, and represent, like the antimeres of the Radiata, individuals 

 of a' lower order, which on the severance of their mutual connec- 

 tion can acquire independence and remain alive for a shorter or 

 longer period (proglottis of Cestodes). 



In animals of higher organization the segments are much more 

 intimately connected, and are mutually dependent, but they lose at 

 the same time their complete homonomy. In the same degree as the 

 metameres acquire an unlike structure, and corresponding to this a 

 varying importance in the life of the organ- 

 ism, they lose their individual independence, 

 and sink more and more to the value of organs. 

 The metameres in the polymorphous 

 colonies are quite analogous to the segments 

 of the individual. In them there follow, one 

 behind the other, similar groups of different 

 individuals, each of which fulfils singly the 

 conditions necessary for existence, and there- 

 fore can continue to live as a colony of a 

 lower order when separated from the stock 

 (Eudoxia , Diphyes, fig. 16). 



The distinction into a higher and lower 

 order also holds for organs. There are organs 

 which are reducible to a single cell, or to an 

 aggregation of equivalent cells (simple organs), 

 and others in the formation of which various 

 cells and tissues (compound organs) partici- 

 pate, and which frequently, in their turn, may 

 be divided into parts different in structure 

 and function. The compound organs of higher order are composed 

 of different parts which function as organs of a lower order. These, 

 again, are composed of various kinds of cells and cell derivates, which 

 are organs of a still lower order. Finally, in the last analysis, we 

 come to the cell or the area of protoplasm corresponding to it, which 

 is the simplest and ultimate organ. On the other hand, we group 

 together organs of different order, which are intimately connected so 

 far as their chief function is concerned, under the name of system 

 (vascular system, nervous system) or apparatus (digestive apparatus), 

 although we cannot clearly distinguish them from compound organs. 



FIG. 1C. Portion of Diphyes 

 after R. Leuckart). D, 

 hydrophyllium ; Gs, gono- 

 phore; P, Polyp with 

 tentacle. The groups of 

 individuals separate them 

 selves as Eudoxia. 



