140 PART I. GENERAL ACCOUNT 



Thus the secondary metamerism of the trunk of Pogonophora is of the same 

 nature as that found in the Enteropneusta and Acraniata, and consequently 

 in the Vertebrata also. A tendency to metamerization of the trunk is evidently 

 characteristic of the Deuterostomia. In the various groups it has appeared 

 independently, involving a variety of organs, but only ever developing in the 

 elongated trunk segment (Ivanov, 1952). 



Evolutionary development within the class 

 When attempting to understand the phylogenetic relationships of the 

 families and genera of Pogonophora, we must place major emphasis upon the 

 structure of the tentacular apparatus and on the degree of secondary meta- 

 merism of the trunk. 



The general body plan is remarkably constant in the various orders and 

 families of Pogonophora, though the tentacular apparatus presents great 

 variability of form. Because of the great physiological importance of the 

 tentacles (p. 84) the evolution of pogonophores must have been bound up 

 from the first with the progressive development of the tentacular apparatus. 

 This appears to be the key organ system in the phylogenetic development of 

 pogonophores (Ivanov, 1960a). Its structure, therefore, is of major importance 

 for the characterization of the different families and orders. 



In general, evolution of the tentacular apparatus has been accompanied by: 



1. A gradual increase in the number (polymerization) of tentacles from 

 6-8 to 200 or more; 



2. An increasing number of absorptive pinnules; 



3. The formation of a permanent intertentacular cavity, through fusion 

 of adjacent tentacles, replacing the temporary cavity of the more primitive 

 forms. 



[It should be noted, however, that the opposite tendencies — towards 

 reduction of tentacles below the primitive half dozen and reduction or loss of 

 the pinnules — have occurred more than once in the evolution of various 

 groups — D.B.C.] 



The origin and development of the metamerism of the adhesive papillae 

 and their cuticular plaques have been set out above (pp. 20-25). Here I will 

 only remark that to all appearances the start of the metamerism and its pro- 

 gressive development have taken place independently in different groups. 

 An equal degree of metamerization in two genera is, therefore, no sign of their 

 relationship. A small number of metameric papillae, however, may be taken 

 as indicating a primitive condition. 



