14. SYSTEMATIC POSITION OF THE POGONOPHORA 137 



It is interesting that these enteropneusts, like pogonophores, possess an egg 

 rich in yolk and lack a pelagic larva. 



The trunk segment or metasoma, moreover, which later becomes the 

 largest part of the body, is at first quite insignificant in size in both Pogono- 

 phora and Enteropneusta. In representatives of both groups the trunk then 

 grows apace in length and in the adult animal it reaches a considerable size. 

 Essentially the same thing happens in the development of the Acraniata. It is 

 well known that in Branchiostoma most of the body is formed by growth in 

 length of the third or trunk segment whose coelom becomes secondarily 

 segmented. 



Finally, the provisional ciliated bands and the ciliated patch on the 

 mesosoma of the embryonic pogonophore must be regarded as rudiments of 

 the ciliary locomotory system found in free-swimming dipleurula larvae. 



Thus all the primitive features of embryological development encountered 

 in Pogonophora are in accordance with the idea of their deuterostomic 

 affinities. 



Many significant features of the structural plan of the adult pogonophore 

 also point to their location amongst the Deuterostomia, and in particular to 

 their relationship with the phylum Hemichordata, in which the chief marks 

 of the trisegmental deuterostomic archetype are preserved in the adult 

 structure. This relationship was remarked by Dawydoff (1948), Ivanov 

 (1955c) and Jagersten (1956) and particularly clearly stated by Beklemishev 

 (1951). The intervening years have seen a great advance in our knowledge of 

 the organization of the Pogonophora and it is now possible to list the follow- 

 ing points of similarity with the Hemichordata : 



1 . The trisegmental body plan ; 



2. The unpaired first coelom; 



3. The pair of coelomoducts by which the first coelom communicates 

 with the exterior; 



4. The possession of a pericardium (though the anatomical nature of this 

 organ remains unclear); 



5. The situation of the gonads in the metasoma; 



6. The possession of a mid-dorsal nerve trunk running the whole length 

 of the body; 



7. The tendency for the dorsal nerve trunk to sink down, in the mesosomal 

 region, to form a neural trough. 



A further similarity between the two groups is the tendency towards 

 secondary metamerism of the elongated trunk segment, though this affects 

 more kinds of organs in the Pogonophora. 



