24 PART I. GENERAL ACCOUNT 



2. Metamerism formed by the differentiation of parts or organs along an 

 originally uniform whole. 



3. The combination of originally independent similar parts or organs into 

 one metameric whole. In this case the metamerism usually arises as a result 

 of the incomplete cross-division of the animal into a series of metamerically 

 disposed sections of the body or zooid. 



As emphasized by Beklemishev, the third type of metamerism is signifi- 

 cantly different from the first two in its initial stages. The metamerism de- 

 velops not in a whole organism originally lacking it, but by means of the 

 association of non-metameric units of a lower order. In the Annelida these 

 units are clearly post-larval segments, forming by division in the zone of 

 growth of the metatrochophore or of the embryo. The metamerism of the 

 Annelida is thus of the third type. 



But from the foregoing it should be obvious that the adhesive papillae in 

 Pogonophora indicate metamerism of the first kind, i.e. a superimposed 

 regulating metamerism. It is equally clear that the metamerism of pogono- 

 phores cannot possibly be considered a result of the combination of origi- 

 nally separate parts of the body. Thus, by its very nature, it has nothing in 

 common with the perfect metamerism or segmentation of the annelids. Nor 

 can we admit the possibility of the second type of metamerism in Pogono- 

 phora, for it is inconceivable that the adhesive papillae arose by chopping up 

 an originally continuous band into short lengths. 



Another interesting feature is the diminution in the number of papillae in 

 the non-metameric part of the trunk as their number in the metamerically 

 organized part increases. To use the terminology of Dogel' (1936, 1954), 

 they become oligomeric. To some extent this happens because a larger and 

 larger proportion of them become involved in the metameric region, which 

 in this way increases in length at the expense of the non-metameric part — a 

 sort of creeping metamerization (Table 1). But this is not the whole explana- 

 tion, for the proportional reduction in the random papillae is greater than 

 the proportional increase in the metameric. 



Though an accurate count of the non-metameric papillae is difficult 

 because of their great variability in number in different individuals, a rough 

 estimate can be made. In all the species with a comparatively small number of 

 paired metameric papillae the non-metameric papillae are very numerous 

 (e.g. Heptabrachia, Polybrachia annulata). In this respect Heptabrachia 

 subtilis is of particular interest, for it has hundreds of adhesive plaques 

 scattered on the dorsal as well as on the ventral side of the greater part 

 of the trunk. But in Spirobrachia beklemischevi the comparatively short 



