88 PART I. GENERAL ACCOUNT 



at the low temperature at which pogonophores live, for they would be swept 

 away before they could operate. It becomes necessary, therefore, to postulate 

 that digestion and absorption of the trapped food take place at a different 

 time and under conditions when the tentacles are enfolded to surround an 

 almost closed cavity through which little or no water is flowing. Now on 

 Jagersten's hypothesis absorption must take place when the tentacles are 

 open to the water and the cilia are maintaining a strong current. The two 

 hypotheses cannot then be complementary and Ockham's razor would 

 demand the abandonment of Jagersten's hypothesis, at least for the time 

 being. 



There remains Ivanov's hypothesis. This clearly accounts for all the 

 anatomical peculiarities of the multitentaculate forms, but what of the 

 unitentaculate Siboglinum, or, indeed, of the bitentaculate genera? Not 

 enough is known of the latter, but it is worth spending a little time examining 

 the former. 



An intertentacular cavity is formed in Siboglinum by the tight helical 

 coiling of the single tentacle, leaving a cavity down the centre. We can 

 imagine such a coiled tentacle protruding from the mouth of the tube with a 

 ciliary current maintained through it, swirling round amongst the pinnules 

 where these are present, for the cilia (where present) certainly beat along 

 the tentacle and hence must maintain a spiral water current. But the mus- 

 cular action needed to maintain the coil of the tentacle unsupported by the 

 tube is difficult to envisage. Many of the smaller species of Siboglinum, 

 moreover, lack pinnules to trap food particles in the way assumed by Ivanov. 

 It is perhaps important in this context that it is only in species of this one 

 genus that pinnules are known to be absent. Should we not then look to 

 another, possibly supplementary, mode of feeding in Siboglinum ? 



Kirkegaard (1961) has attempted to observe feeding in living S. ekmani 

 maintained at low temperatures in the laboratory. He found that they were 

 sensitive to light and remained withdrawn in the tube when he attempted 

 visual observations. But like many nocturnal marine animals they are in- 

 sensitive to red light and although observations of such a fine object as the 

 tentacle of S. ekmani is difficult in dim red light it is nevertheless possible. 

 Under such conditions, in healthy animals the tentacle may be seen protrud- 

 ing from the tube, not coiled as Ivanov's hypothesis would demand but 

 extended and questing over the surface of the surrounding substratum like 

 the arm of a blind man groping for food. On at least two occasions an animal 

 has been seen to pick up a particle, perhaps a rhizopod or perhaps a particle 

 of detritus, coil the tentacle around it and retreat into the tube. 



