or^ 



!."*^ 



^■^/X 



THE NEW EVOLUTION 



A^* 



^et:»^ though alwaj^s ,,^j^ 



Fig. B. — Although all coelenterates are developed geometri- 

 cally through a gastrula or strictly comparable stage, few of 

 them are perfectly radial in their symmetry. Many of them have 

 a slit-like mouth instead of a circular mouth, with the two halves 

 of the animal on either side of a plane passing through the long 

 axis of the slit-like mouth alike. This condition is known as 

 "biradiate" symmetry, and there are in the coelenterates all tran- 

 sitions between truly radial and "biradiate" symmetry. But all 

 coelenterates are either radial or "biradiate," and all agree in 

 having the body composed of only two germ layers, ectoderm and 

 endoderm; there is no intermediate or mesodermal layer. 



From this condition of radial, or approximately radial, sym- 

 metry and no mesoderm there are two lines of departure. Both 



[140] 



