222 



GEOLOGICAL BIOLOGY. 



phology, we observe that in attaining differentiation from this 

 simple state several systematic groups of differences are 

 expressed. The first is concerned with general direction of 

 motion, expressing itself in the arrangement of the body 

 shape, or in its development. 



Polarity. — If we imagine the primitive form to be a globe, 

 its motion is expressed by assuming polarity of direction — 

 a definite anteriority, or direction toward which motion ap- 

 proaches, and the opposite, posteri- 

 ority, from which it goes. Every 

 living animal having reached the 

 first stage of differentiation (seen 

 in the Metazoa, as the Coelenterata, 

 for instance) expresses some degree 

 of polarity. The longitudinal axis 

 of the body, in the Metazoa of this 

 simplest form, is clearly expressed, 

 and the anterior end is primarily 

 determined by the position of in- 

 vagination in the 

 embryo forming 

 Thus the simple 



growth of the 

 the gastrula. 

 coral polyp is 



Fig. 52. — A simple coral polyp (il/f 

 iridium ma7-ginatum Les.), rep 



resentiiiK the gastrula stage of dif- a iTiaturc aiiimal representing the 



ferentiation, in which the posterior •■■ '-' 



end of the body /y is attached and (^^_y/;'/^/^ sta(;;e of eiiibryonic de- 



the anterior end A is free. " •' 



velopment of higher animals. 



In Fig. 52 the anterior end of the axis of the body, 

 AB, is at A, which is the mouth or oral end of the enteron 

 or digestive cavity. This is the centre of the free end of the 

 body, and the opposite end, B, is in mature stage often fixed. 



Antinieres and Metameres. — As such an organism is sup- 

 posed to develop parts by differentiation, these parts are 

 arranged in one of the following three ways : radially, or 

 around the axis, when they are called Aiitimercs ; or one 

 after the other in the direction of the long axis, when they 

 are called Metameres ; or, third, without repetition of parts, 

 except to express bilateral symmetry and a dorso-ventral 

 opposition of parts. 



Radiate Structure, Bilateral Symmetry, and Actinimeres. — The 

 primary axis [AB in Fig. 52) is the one which is longitudinal 



