8 THE EVOLUTION OF THE METAZOA 



former has been inherited from the protozoic ancestors, the 

 latter corresponds to a disposition which is common to all 

 metabionts including metaphytes to develop a multicellular 

 organism from a zygote (see the conditions in Volvox, a green 

 alga). 



In this way the second pillar of the old concept about Coelen- 

 terata collapsed with the definite removal of Spongiae from the 

 relationship with Coelenterata. This was initially made by 

 Leuckart (1859). The first pillar was the removal of Hydra 

 from the basis of Hydrozoa. 



The Position of Ctenophora in the Animal Classification 



It was comparatively easy to show that the removal of Spon- 

 giae from the vicinity of Cnidaria could be convincingly 

 upheld; such was their isolation (as Parazoa) from all other 

 Metazoa. The great differences that exist betw^een Spongiae 

 and the remaining Coelenterata— the latter taken in the old 

 sense of the word— are evident since they are both considerable 

 as well as numerous. As has been already emphasized, we have 

 here two completely different "spirits of organization" which 

 force us to consider a different origin. It is clear that though 

 there are some similarities between Spongiae and Cnidaria, that 

 these similarities are purely external and can be mostly traced 

 back to a sessile way of Hfe and to a generally low stage of 

 organisation. 



Considerably greater difficulties have to be overcome if we 

 try to exclude Ctenophora from the triangle Spongiae-Cni- 

 daria-Ctenophora. It is clear that Ctenophora stand much 

 closer to Cnidaria. Both are animals whose digestive organs 

 have reached the aproctous stage of development ; they have 

 one opening only, the oral opening. In both, the digestive 

 tract shows an inclination to the formation of the so-called 

 gastrovascular system. Due to their life as plankton, a similar- 

 ity of forms developed, particularly in medusae and Cteno- 



