646 The Animal Kingdom 



diphyletic theory. According to this, immediately after the development 

 of bilaterality two main branches of development occurred. One of 

 these, the Deuterostomia, culminates in the chordates, the other, the 

 Protostomia, in the arthropods. These branches are separated from one 

 another on several fundamental differences. The members of the Pro- 

 tostomia branch have determinate cleavage, a mouth that develops from 

 the blastopore or in the region where the blastopore closed, the mesoderm 

 developing from an ingrowth of cells from the ectoderm, and often there 

 is a trochophore type of larva. The members of the Deuterostomia 

 branch have indeterminate cleavage, the anus developing from the blasto- 

 pore, and the mouth representing a new structure, the mesoderm formed 

 by outfoldings of the archenteron, and finally there is often a type larva 

 known as dipleurula or echinoderm type. 



Fig. 210 shows the phylogenetic relationships of the various phyla 

 according to this theory. 



ARTHROPODA 



OTHER 

 SCHIZOCOELA 



ECHIUROIDEA 



NEMERTINA 



PLATYHELMINTHES 



ASCHELMINTHES 



o 



^§ 



DC 

 CL 



PRIMITIVE ACOELA FLATWORMS 



bilat'eria 



PORIFERA 



OTHER PROTOZOA 



FLAGELLATA 



Fig. 210. — Diagram of the relationships of the phyla. (By permission from The 

 Invertebrates by Hyman, copyright 1940, by McGraw-Hill Book Company, Inc.) 



Origin of the Vertebrates. — From the above theory, it appears 

 that the phyla composing the Protostomia are actually quite clearly re- 

 lated to one another. Unfortunately the Deuterostomia branch which 



