6 PRIMITIVE ANIMALS 



a necessary condition for movement directed in a 

 straight line it has led to the concentration of sense 

 organs with their nerve-centres in a forwardly 

 directed head, and to the disposition of all the im- 

 portant organs in pairs on either side of the median 

 axis along which the animal moves. In practically 

 all free-living and active animals the bilateral sym- 

 metry is retained, and wherever it has been lost, as 

 in many parasitic and fixed forms, we can always 

 perceive that the loss is a result of the lack of 

 necessity of movement. 



The Platyhelmia differ from the Coelenterata not 

 only in their type of symmetry, but in possessing a 

 definite cell-layer, the mesoderm, interposed between 

 the outer ectoderm and the inner digestive endoderm. 

 Within this middle layer, from which the muscular 

 and supporting structures are formed, the repro- 

 ductive cells are always developed as hollow sacs 

 lined with a fairly regular epithelium or investment 

 of cells (Fig. 2). 



We do not know by what steps the bilaterally 

 symmetrical Metazoan with three germ-layers and 

 saccular reproductive organs in the mesoderm was 

 derived from the Coelenterate type, though certain 

 Coelenterates have taken to a creeping habit and 

 have developed a bilateral symmetry. These animals 

 are, however, probably highly specialized Coelente- 

 rates, and do not represent real intermediate forms, 



