NO. 6 ANNELIDA, ONYCHOPIIORA, AND ARTHROPODA SNODGRASS 3 



proliferation of cells from the posterior pole of the blastula (D, E, F), 

 and this last process suggests that it is an embryonic modification 

 of gastrulation by invagination. When, shortly, the planula settles 

 to the bottom of the water and becomes attached preparatory to its 

 metamorphosis into a polyp, a stomach cavity appears in the endo- 

 derm, and a mouth cavity breaks through at the free pole. 



The development of the planula shows clearly that there is in 

 ontogeny no fixed method for the formation even of so important 

 an organ as the stomach. The effective thing in embryonic develop- 

 ment is the inherited organizing property resident in the egg that 

 converts a mass of cells, however formed, into a definite functional 

 structure. The same principle, as we shall see later, applies also to 

 the development of the annelids and the arthropods, for in these 

 animals there is so much apparent irregularity in the formation of 

 the germ layers that attempts to interpret all observed facts in terms 

 of cell genealogy lead only to confusion. Ontogeny and phylogeny, 

 therefore, while they produce the same end results, may follow quite 

 different methods of procedure. In phylogeny we must visualize the 

 successive stages in the evolution of an animal as free-living adult 

 forms, each structurally adapted for performing the functions of an 

 independent animal. 



If the coelenterate planula were an adult animal instead of a 

 temporary larval form, or if it had to maintain itself for any con- 

 siderable length of time, it almost certainly would have a stomach 

 cavity and a mouth. Thus modified, as it is later in its own develop- 

 ment, the planula would be an independent, motile gastrula, having 

 a stomach in the form of an open pocket of the blastoderm for the 

 retention of food particles (fig. 2 A). An animal of this simple type 

 of structure, we must suppose, was the actual ancestor of the polyp 

 and medusa forms of the Coelenterata ; but equally well it might have 

 been the progenitor of the annelids, and through the latter of the 

 arthropods. Various writers on phylogeny have proposed an origin of 

 the segmented worms direct from a coelenterate polyp, but it should 

 be recognized as a fundamental principle in evolution that a special- 

 ized type of animal does not give rise to another specialized type — if 

 two forms are related, they are related through some simple common 

 ancestor. This principle as applied to the coelenterate derivatives is 

 expressed by Ziegler (1898), who says: 



It is to be supposed that the higher animals derived from coelenterate stock took 

 their origin not from the highly specialized forms of the Coelenterata, such as 

 the anthozoans and ctenophores, but from a planula-like or gastrula-like ancestral 

 form of the coelenterates. 



