MORPHOLOGY OF ANIMALS 



differentiation of axes and poles. This can actually be seen 

 in the embryology of certain animals where radial symmetry 

 develops into bilateral, and the latter into asymmetry. On 

 the other hand, in the case of some bilateral animals the 

 eggs even are bilateral. 



The chief axis of the adult connects the anterior and 

 posterior poles and is called the antero-posterior axis. The 

 side of the body which is usually directed downward is 

 ventral, the opposite side dorsal, and the axis connecting 

 these is the dorso-ventral axis. The axis connecting the right 

 and left sides of the body is the transverse axis. A plane 

 passing through the body in the antero-posterior and the 

 dorso-ventral axes and dividing the body into equivalent 

 right and left halves is called the median (bilateral) plane 

 or section. A plane passing through the dorso-ventral and 

 the transverse axes is a transverse plane, or section, while a 

 plane passing through the antero-posterior and the trans- 

 verse axes is called a coronal (frontal) plane or section. 



C. MORPHOLOGY OF METAZOA 



Metazoa are animals composed of many cells which are 

 arranged in at least two layers, an outer layer covering the 

 body, the ectoderm, and an inner layer lining the digestive 

 cavity, the endoderm ; in addition most metazoa have a mid- 

 dle layer or aggregation of cells, the mesoderm, lying be- 

 tween the ectoderm and endoderm. All have male and 

 female sex-cells (spermatozoa and ova). 



I. STAGES OF METAZOAN DEVELOPMENT 



In the course of development all metazoans pass through 

 the following stages (Fig. i ) : 



C 12 3 



