CHAPTER 2 



External Morphology and the General Body Plan 



The general body plan of Pogonophora 



The complete absence of a gut in the Pogonophora in the adult condition, 

 though it appears so surprising, is already well established and has been 

 verified in many forms (Caullery, 1944; Ivanov, 1952, 1955a, 1960a; 

 Jagersten, 1956). Until very recently the question of how pogonophores feed 

 seemed altogether mysterious. Not surprisingly, therefore, Caullery (1944) 

 assumed that the specimens at his disposal must represent torn-off zooids 

 from polymorphic colonies and that it was by chance that no gastrozooids 

 had been collected. This hypothesis, advanced by Caullery in a very guarded 

 manner, seemed to fit the facts, and even as recently as 1956 Jagersten sub- 

 jected it to serious discussion, not arriving, however, at any definite con- 

 clusion. 



The investigation, however, of more and diverse material clearly rules out 

 any idea of a colonial nature in the Pogonophora. In a series of complete 

 specimens (two species of Lamellisabella, Oligobrachia dogieli, Heptabrachia 

 beringensis and several species of Siboglinum) the hind end of the body has 

 been described (Ivanov, 1952, 1957a), which completely eliminates any 

 possibility of interpreting individual pogonophores as zooids from a colony. 

 The problem of their feeding has also been decided; as shown by an analysis 

 of the structure, the Pogonophora are capable only of external digestion, 

 carried out by means of the tentacular apparatus (Ivanov, 1955b). This 

 immediately removes all grounds for the colonial hypothesis. 



The other major problem of the architectonics of pogonophores is the 

 question of the orientation of the body. Since the embryological development 

 has been inadequately studied, and the mouth and gut are absent in the adult, 

 it is difficult to decide which is the ventral and which the dorsal side of the 

 body. In the present work, as in my earlier papers and in accordance with the 

 opinion first advanced by Johansson (1939), the dorsal side of the body is 

 taken to be that on which lie the cephalic lobe, containing the brain, and the 

 tract of ciliated epithelium (p. 18), and along which runs the median nerve 

 tract. The ventral side is accordingly taken to be that to which the tentacles 

 are attached and along which are disposed the metameric adhesive papillae 

 of the trunk (p. 45). 



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