8. THE TENTACULAR APPARATUS AND EXTERNAL DIGESTION 87 



Thence they pass into the vessels of the tentacle and to the dorsal vessel to be 

 distributed around the whole body.* Perhaps the filtration of water by the 

 tentacular apparatus and the digestion of food do not proceed simultaneously; 

 that is to say, first a supply of food is amassed and then the animal digests it 

 with the tentacular crown withdrawn into the tube. This hypothesis, based 

 for the moment only on the results of morphological studies, stands in need 

 of experimental confirmation. It offers an explanation, however, for such 

 enigmatic features as the complete absence of any gut despite clear evidence 

 of a continual abundant intake of food, the peculiar structure of the tentacular 

 apparatus, and the characteristic development of the blood system [which is 

 exceptionally complex for so small an animal]. 



Thus the Pogonophora represent a unique and complete exception among 

 free-living non-parasitic animals in lacking a gut and possessing external 

 digestion. 



In 1957 Jagersten pointed out the possibility of another solution to the 

 problem of their feeding mechanism. In his opinion one cannot exclude the 

 possibility that the tentacles absorb dissolved amino acids, which, perhaps, 

 form in the surface layers of the mud by bacterial action. The acceptance of 

 this hypothesis, however, leaves a whole series of peculiarities of the ten- 

 tacular apparatus unexplained, and in particular the facts that the pinnules 

 are always so located that they are turned towards the inside of the tentacular 

 crown (or, in other words, they lie within the intertentacular cavity) and that 

 an intertentacular cavity is formed by the fusion of one tentacle to the next in 

 the highest forms. Moreover the presence of free amino acids in sea water in a 

 quantity sufficient for the nutrition of animals by means of absorption is 

 extremely doubtful. 



[It is perhaps worth pointing out that Ivanov's and Jagersten's hypotheses 

 are mutually exclusive, i.e. we cannot assume that pogonophores might 

 supplement a solid diet by the absorption of dissolved food substances. For 

 Jagersten's hypothesis demands that food be absorbed while the tentacular 

 crown is extended, while Ivanov's demands a cycle of activity in which 

 absorption takes place in the retracted crown, the expanded crown serving 

 only for collecting food. It cannot be doubted that the basic metabolism of 

 Pogonophora is high and that they must have a high rate of food capture. 

 If Ivanov's hypothesis is correct, therefore, the ciliary current down through 

 the intertentacular cavity must have a high velocity. Under such conditions 

 secretion of digestive enzymes into the cavity would be useless, particularly 



* N. G. Khlopin has remarked that the Pogonophora may be designated "Brachiotropha" 

 (Gr. fip&xlwv — arm, Tpo<f>6s — feeding). 



