CHAPTER 8 

 The Tentacular Apparatus and External Digestion 



Both Caullery (1914, 1944) and Johansson (1939) remarked on the absence 

 of any gut in Pogonophora, and in later years this astonishing fact was con- 

 firmed in species after species (Ivanov, 1952, 1955a, 1956b, 1960a; Jagersten, 

 1956). Mouth and anus are likewise missing and do not appear even during 

 embryonic development (Ivanov, 1957b). 



Pogonophora must, however, require a large and steady supply of food. 

 This is apparent from their high basic productivity, as evinced by the great 

 accummulation of yolk and fat in the large eggs, by the intensive secretory 

 activity of the numerous multicellular glands which pour out the materials 

 for the very long and often thick-walled tube, and by the formation of new 

 tentacles which often continues even in the sexually mature animal. [The 

 possession of a closed circulation and pigmented blood also implies a high 

 metabolic rate.] 



The key to the problem of how these sedentary inhabitants of the sea bed 

 feed, though wholly lacking the usual digestive system, is given by an analysis 

 of their tentacular apparatus. 



The structure of the tentacles 



Except for a few species of Siboglinum, which lack pinnules, the structure 

 of the individual tentacles is very uniform throughout the Pogonophora. 

 The greatest known modifications of the tentacles are found in Lamellisa- 

 bella zachsi and Spirobrachia grandis (Ivanov, 1955b, 1960a). 



The wall of the tentacle consists of a hollow cylinder whose outer layer is 

 formed of more or less tall epidermal cells. Under the basement membrane 

 of these cells lies a single layer of very fine muscle fibres, then comes the 

 peritoneal endothelium lining the coelomic canal. The afferent and efferent 

 blood vessels run in the coelom (Figs. 53, 55). The surface of the tentacle is 

 covered by a cuticle which is thicker on those sides which are on the outside 

 of the crown. The tentacular nerve runs up the outer side of the tentacle in 

 the thicker epidermis there, always, however, lying a little to one side, 

 asymmetrically (Fig. 55). On the side which faces into the crown the surface 

 of the tentacle bears a row of pinnules like a fringe (p. 31). The pinnules are 

 flanked on each side by a single row of gland cells and then by a single row of 

 large ciliated cells outside these, before the cuticle begins (Fig. 55). 



79 



