2. EXTERNAL MORPHOLOGY AND THE GENERAL BODY PLAN 27 



In the polytentaculate Poly brae hia and Galathealinum the primitive 

 horseshoe is retained, but the tentacles are disposed not in one but in two 

 or even three rows, e.g. in Polybrachia annulata (Fig. 17C). This feature is 



ftMfc 



% & 



Fig. 17. Diagram of arrangements of the bases of the tentacles in various species of Pogonophora. 

 A - Oligobrachia dogieli; В - Birsteinia vitjasi; С - Polybrachia annulata; D - P. barbata; 

 E - Lamellisabella zachsi; F - Spirobrachia grandis. (After Ivanov, 1960a.) 



even more strongly marked in P. barbata (Fig. Y1D) and in Galathealinum 

 bruuni in which the tentacles are many rows deep in the horseshoe base of 

 the crown. 



A quite different phylogenetic path of multiplication of the tentacles has 

 taken place in Spirobrachia. Here the tentacles remain in a single row in the 

 crown but the right-hand end of the horseshoe is greatly extended and 

 twisted into a dextral centripetal spiral (Fig. Y1F). In this way the uniserial 

 arrangement of tentacles is preserved while at the same time allowing for a 

 great increase in their number as they form an asymmetrical spiral crown. 

 But even this proved inadequate and the necessity for yet more tentacles has 

 been satisfied by the development of a new structure, a corkscrew-like 

 helical extension of the protosoma, the lophophore, lying within the base of 

 the crown and bearing the greater part of the tentacles (Fig. 18). In this way 

 the right half of the crown has become hypertrophied in Spirobrachia 

 (Ivanov, 1952, 1957a). 



