136 PART I. GENERAL ACCOUNT 



used in place of the name Deuterostomia the term preferred by many 

 English authors — "the echinoderm-chordate stem". In her opinion the fate 

 of the blastopore does not sufficiently characterize the Protostomia and 

 Deuterostomia, since within the same stem there are groups with different 

 blastoporal fates. The same might be said about such features as the nature 

 of cleavage and the mode of formation of the coelomic mesoderm. It must be 

 stated, however, that all these exceptions to the general rule are merely the 

 results of secondary changes. In other words it is impossible to deny that the 

 fundamental ontogenetic pathway of all the primitive Deuterostomia was : 

 radial indeterminate cleavage, the blastopore associated with the anus, 

 secondary formation of the mouth and enterocoelic origin of the coelom; 

 and in the structure of the adult the common feature has been the triseg- 

 mental body plan. [The precise term we use to express this fundamental 

 unity is immaterial, but] there is no real basis for abandoning the terms 

 Deuterostomia and Protostomia. 



The current status of our knowledge of the embryological development of 

 the Pogonophora (Ivanov, 1957b) allows us to draw a few conclusions about 

 the systematic position of the group despite the many gaps in our knowledge. 

 As we have seen (pp. 103-119), the embryology of pogonophores in general 

 shows clear evidence of secondary modification — in the bilateral symmetry 

 of the egg and cleavage stages, the lack of an invaginated gastrula and corre- 

 spondingly of a blastopore and an epithelial archenteron, and in the absence 

 of a pelagic larval stage, so that development is direct. Even the earliest 

 cleavage stages give no information about the primitive mode of cleavage : by 

 the third division, when it usually first becomes clear whether cleavage is 

 spiral or radial, Siboglinum has already diverged sharply from either of these 

 modes of development. 



Primitive features are, however, still preserved in later stages of develop- 

 ment allowing one to compare the embryogenesis of Pogonophora with that 

 of some other animals. Thus the coelomic mesoderm forms by enterocoely 

 and in particular closely resembles the formation of the coeloms in those 

 Enteropneusta which have an egg rich in yolk (Saccoglossus pusillus). This 

 alone argues strongly in favour of assigning the phylum Pogonophora to the 

 Deuterostomia. An essential point of similarity with the Deuterostomia lies 

 in the fact that the protocoele is unpaired from the very beginning. 



In place of a dipleurula larva Pogonophora produce an elongated embryo 

 whose body is quickly dissected into three segments. In general it recalls the 

 early stages of metamorphosis of Enteropneusta, particularly in those forms 

 with direct development, such as Saccoglossus kowalewskyi and S. pusillus. 



