2. EXTERNAL MORPHOLOGY AND THE GENERAL BODY PLAN 23 



We may note that in all species the number of metameric papillae is not 

 fixed but is, on the contrary, subject to very great individual variation. It is 

 interesting also that as the number of pairs of metameric papillae increases 

 their metamerism does not become more regular and perfect. In this respect 

 the lower forms are not distinguished from the more advanced, and in the 

 one as in the other, numerous irregularities and considerable individual 

 idiosyncrasy are displayed in the arrangement of the metameric papillae. To 

 speak of a paired arrangement of the papillae is to use a merely relative term. 

 This imperfect metamerism is sufficiently characteristic of and peculiar to 

 the Pogonophora to stand out in comparison with the segmentation of the 

 polymeric Annelida, for instance the Polychaeta. The metamerism of the 

 latter, as is well known, is distinguished by its great regularity and steadiness. 

 It could not be otherwise, for it is an inevitable expression of the very method 

 of development of the segments during ontogeny. The post-larval segments, 

 originating one behind the other in the sub-terminal zone of growth of the 

 metatrochophore form identical sets of organs (P. P. Ivanov, 1928, 1937, 

 1944). It is natural, therefore, that the metamerism of the post-larval section 

 of the body of the Annelida is one of the most highly developed and perfect, 

 and in comparison the metamerism we encounter in the Pogonophora appears 

 to be of a different nature. 



It long ago became apparent that a metameric disposition of the organs of 

 an animal may be the result of totally different processes. Thus Meier (Meyer) 

 (1890) and Lang (1903) admitted the possibility of the division of the whole 

 body of the animal by metamerism in those cases when a metameric arrange- 

 ment of the organs presented some physiological advantage (e.g. in con- 

 nexion with locomotion). On the other hand, according to Hatschek (1878), 

 the metamerism of the annelids arose merely as a result of the ability of the 

 hypothetical ancestor (the "trochozoon") to form, on the hind end of the 

 body, small uniform parts, which could develop either as separate individuals 

 or as segments of a common organism. Quite recently the question of the 

 pathways by which metamerism could develop was examined in detail by 

 Beklemishev (1952), who investigated at length the manifestations of 

 metamerism throughout the animal kingdom. Beklemishev distinguished 

 the following three modes of origin of metamerism : 



1. Metamerism regulating an originally disorderly disposition of similar 

 parts or organs. Metamerism arising in this way will not become perfect and 

 regular or firmly stabilized, and is characteristically incomplete. Not in- 

 frequently a certain lack of agreement is apparent between the metamerism 

 of different organs. 



