256 INVERTEBRATE ZOOLOGY 



characters of the embryonic gastruhi. Though our present-day 

 Coelenterata are said to stand on a level of the gastrula in funda- 

 mental structure, the Gastraea must not be confused with the 

 members of this phylum which have undergone great differentia- 

 tion and progressive evolution. 



Without discrediting the Gastraea theory in the least, it 

 represents but a step in the right direction, for do not the various 

 gastrulae in turn originate from a single-celled condition — ^the 

 fertilized egg? Thus all animal forms may be traced along their 

 phylogenetic paths to a common ancestry in the one-celled 

 organisms. Even Haeckel recognized this fact at the time when 

 he propounded his Gastraea theory. The highways between 

 the single-celled condition and our highly diversified Metazoa 

 have not all been surveyed. Only here and there are there 

 indications of the routes which have been taken. Some forms 

 seems to have gone on an independent track early in the course 

 of evolution. Others seem to have traveled long distances 

 together before the ancestral stock became diversified to form 

 various ones of our present-day animals. Some groups, such 

 as the Vertebrata, the Echinoderma, and the Nematoda, seem 

 to have left no conclusive evidence regarding their relation- 

 ships to any other animal groups. Speculation plays a large 

 part in attempting to decipher the faint lines which are directed 

 toward but never lead to any of the usual roads of descent. 



In the following section attention will be directed to some of 

 the probable ancestral lines of the various Metazoa. 



Metazoan Tendencies in the Protozoa. — In Chapter II, it has 

 been pointed out that within the Protozoa are found many 

 indications of tendencies toward metazoan conditions. Chief 

 among these are colony formation and isolation of germ plasm 

 from the soma. Much has been made of the parallel between 

 the blastula stage in the embryology of the Metazoa and the 

 spherical arrangement of the cells in colonies such as Volvox. 

 By one or several paths of descent, the ancestors of our present- 

 day Protozoa have probably been the direct or indirect source of 

 all the remaining phyla. 



Porifera. — While the Porifera are usuallj^ accorded a position 

 as the lowest phylum of the Metazoa, there are numerous 

 reasons for assuming that the simplicity of sponges is attributable, 

 in part, to degeneracy or to regressive evolution. The collared 

 cells of the Porifera are the exact counterparts of the bodies of 



