14. SYSTEMATIC POSITION OF THE POGONOPHORA 



143 



of the tentacles. [This seems to have taken place independently in the two 

 genera.] 



The evolution of the rest of the Polybrachiidae is characterized chiefly by 

 the progressive multiplication of the tentacles and consequent changes in the 

 arrangement of the horseshoe base of the tentacular crown (p. 26). Thanks to 

 the increase in the number of tentacles and in the number of longitudinal 



■I» -^ 



Oligobrachiidae 



Siboglinidae 



Athecanephria Thecanephria 



Fig. 88. Diagram of the phylogenetic relationships of the Class Pogonophora, showing also the 

 evolution of the base of the tentacular crown. 



rows of pinnules on each of them, the functional efficiency of the tentacular 

 crown is enhanced; its filtering capacity is improved and the absorptive 

 surface is greater. All Polybrachiidae, except Diplobrachia [and Sclerolinum], 

 fall into a morphological series in which the number of tentacles increases 

 from five to eight in Heptabrachia to more than 200 in Galathealinum 

 brachiosum. But the genera of the Polybrachiidae do not form a single line of 



