THE ANIMAL KINGDOM 5 



one for the Metazoa, since it is adhered to with 

 extraordinary uniformity in all the subsequent de- 

 velopments of the animal kingdom, and where it 

 has been lost, as in the Echinoderms or Star-fishes, 



Fig. 1. Transverse section through a Coelenterate. al. alimentary 

 canal or gastro-vascular cavity, end. endoderm. ect. ectoderm. 

 mesogl. mesoglaea. 



which possess a radial symmetry, the loss is evidently 

 secondary, these animals, as their development shows, 

 being derived from bilaterally symmetrical ancestors. 

 The bilateral symmetry of the Metazoa, from the 

 Platyhelmia upwards, has exercised a profound in- 

 fluence on their evolution. Acquired originally as 



