14. SYSTEMATIC POSITION OF THE POGONOPHORA 141 



When we consider the relationship between the different groups of Pogono- 

 phora we must pay attention to many other features of organization, such as 

 the secondary fusion of the protosoma with the mesosoma. Such fusion is 

 certainly a manifestation of the growing integration of the more specialized 

 higher forms, but it has arisen independently in different phyletic lines. 



Of the two orders of Pogonophora the Athecanephria must be accounted 

 the more primitive. Their tentacular apparatus consists of a few free tentacles 

 each bearing no more than two rows of pinnules. They possess a pericardium, 

 which is lost in the Thecanephria. Physiological contact between the excre- 

 tory coleomoducts of the protosoma and the blood system is less perfect 

 than in the Thecanephria and takes a more primitive form (pp. 66-70). 



The most primitive known athecanephrian genus is, without any doubt, 

 Oligobrachia. The small number of free tentacles, the external division 

 between the protosoma and the mesosoma and whole series of other features 

 vouch for this. Oligobrachia, moreover, has no sign of any metameric papillae 

 whatever in the preannular region of the metasoma, and the metameric 

 dorsal glandular patches on the postannular region of the trunk are hardly 

 discernible. [Perhaps the nearest genus to Oligobrachia is Nereilinum, which 

 likewise has no metameric papillae in the anterior part of the trunk. It differs 

 from Oligobrachia chiefly in the secondary reduction of the number of 

 tentacles to two. The third genus of the family Oligobrachiidae — Birsteinia — 

 differs from Oligobrachia in the opposite way, that is to say by increasing the 

 number of tentacles.] Oligobrachia has six to nine, Birsteinia vitjasi has 12 

 tentacles. They have a series of features in common which suggest that they 

 should be included together [with Nereilinum] in the one primitive family 

 Oligobrachiidae (Ivanov, 1957b). Birsteinia is distinguished chiefly in the 

 development of metamerism in the anterior part of the trunk. B. vitjasi has 

 50-55 pairs of metameric papillae furnished with weakly developed cuticular 

 plaques. 



Well-developed metameric papillae are found also in [the second athecane- 

 phrian family — the Siboglinidae]. Thus Siboglinum caulleryi, for example, 

 has 50-75 pairs of papillae (without plaques). Birsteinia and the Siboglinidae, 

 however, represent independent branches of development, for the former 

 genus characteristically has a number of tentacles while [Siboglinoi'des has 

 only a single pair and] Siboglinum has lost all but the first right tentacle 

 (p. 25, 116). Thus the metamerization of the papillae in these two lines has 

 taken place independently. [The reduction in tentacle number to two also 

 seems to have taken place independently in Nereilinum and Siboglinoldes 

 (Ivanov, 1961a)]. 



F* 



