PHYLOGENETIC RELATIONS OF ANIMAL GROUPS 865 



such colonies, e.g., volvox, become specialized as gametes for repro- 

 ductive purposes and the other cells remain as somatic cells. Be- 

 cause of the similarity between various mastigophoran colonies and 

 blastula stages in metazoan development, this group has frequently 

 been cited as the predecessors from which Metazoa at some time 

 originated. 



Development of the Gastrula 



The primitive gastrula is thought to have had its origin from a 

 spherical colony of Protozoa by a more rapid growth of the cells 

 at one pole which brought about an infolding or invagination to 

 produce a new cavity from the exterior. This development estab- 

 lishes a body composed of two general layers. Modern adult coe- 

 lenterates, like hydra, and others demonstrate precisely this condi- 

 tion today, with the outer ectoderm and the inner endoderm, each 

 composed of cells serving vital general functions. This allows all 

 cells surface exposure either directly on the exterior or to the cavity. 

 By this means larger and more complex organisms were developed 

 and have lived. 



Trochophore Larva 



In Platyhelminthes, Annelida, Molluscoidea, Trochelminthes and 

 Mollusca there occur larval forms of the trochophore type. Al- 

 though the surface ciliation and some other superficial features 

 differ somewhat, a direct relationship among these forms is trace- 

 able through the larva. The fundamental morphology of the cteno- 

 phore is similar to that of the trochophore larva, and the Platy- 

 helminthes are generally thought to have been derived from Cteno- 

 phores. The regular arrangement of gonads and the even, orderly 

 distribution of the diverticula from the intestines of the flatworms 

 are interpreted by many to indicate preparation for segmentation 

 as it appears in annelids. 



Peripatus and the Wormlike Ancestry of Arthropoda 



Peripatus, the only representative genus of class Onychophora, was 

 considered a segmented worm for a long time because of its shape, its 

 even, conspicuous segmentation, and possession of nephridia for ex- 

 cretion. But later it was discovered that the respiration is carried 

 on by tracheae, and the body cavity serves as a blood space, both 

 of which are typical arthropod features. In addition, the paired 



