TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM. 235 



complementary functions, the individual is not a compound 

 organism, but is always cellular. All such animals are 

 grouped under the one division, Protozoa. 



When cells divide, so that as cells they are separate, but 

 remain in close association, with division of labor, one func- 

 tion played by some, another function performed by others, 

 the result is tissue and organ, and the individual is an organ- 

 ized individual ; this constitutes the group of Metazoa. 

 Classifying these Metazoa on the basis of the direction along 

 which development of the specialized parts of the body pro- 

 ceeds, we find the same grand division appearing prominently 

 before us as types of construction of the complex organism. 



The Polymeric Type. There are two fundamental direc- 

 tions along which their development proceeds. Taking the 

 mouth, the opening or entrance to the enteron, as the centre, 

 multiplication of parts may be around this centre (radiate), 

 or it may be from it in the direction of the axis of the 

 enteron (longitudinal) ; and besides these two ways there is 

 probably no other direction of multiplication of parts. When 

 the multiplication is indefinitely radial, it produces the antim- 

 eres of the coral polyp, having chambers and tentacles dis- 

 tributed about the mouth as tentacles, and extending back- 

 ward as septae in the body-cavity. The Ccelenterata ex- 

 press this mode of construction in its most indefinite manner; 

 the Echinodermata express it with definiteness of number of 

 radiations but with a tendency to the following type, and this 

 may be named the polymeric type of construction. 



The Dimeric and Monomeric Types. The second path is by 

 the specialization of the polymeric types with limitation of mul- 

 tiplication to repetition in two opposite directions, forming 

 bilateral symmetry. This is seen in some of the Mollusca, 

 as in Fig. 35, representing the idealized primitive Mollusk. 

 This may be called the dimeric type of construction. 



In a third case there is no full duplication of organs, 

 although the motion and the form are as in the dimeric type. 

 This may be called the monomeric type. 



The Cephalopoda are in part polymeric, but in the main 

 monomeric. The Molluscoidea are polymeric and monomeric 

 in different parts of the body. Thus the branches Ccelen- 



