2. EXTERNAL MORPHOLOGY AND THE GENERAL BODY PLAN 3 1 



Lamellisabella, which has a ring of tentacles (Fig. 17£), this results in the 

 formation of a long hollow tentacular cylinder (Figs. 2, 58), while in Spiro- 

 brachia a tentacular plate is formed which is twisted into a spiral, not unlike 

 a rolled up newspaper (Figs. 18, 59). 



Each tentacle, except on its basal part, bears numerous very fine delicate 

 filiform pinnules, almost meriting the name of "villi". They are rarely more 

 than 750/x long and are found on the inner surface of the tentacles so that 

 they always jut out inside the crown into the intertentacular cavity. They are 

 arranged in one or, more often, in several dense longitudinal rows, alternating 

 with each other in adjacent rows. Thus in one group of species of Siboglinum 

 (e.g. 5. caulleryi and S. cinctutum) the pinnules of the tentacle form only one 

 row, in which, however, may be seen the rather faint traces of a biserial 

 arrangement (Fig. 3). Other species of this genus (S. plumosum, S. ekmani 

 etc) are characterized by two rows of pinnules (Fig. 195), a condition found 

 also in Oligobrachia and Birsteinia. In Lamellisabella zachsi the pinnules form 

 three rows and in Spirobrachia grandis four rows. It should be remarked that 

 in a whole series of species of Siboglinum the tentacle lacks pinnules. Thus 

 according to the observation of Southward and Southward (1958b) there are 

 no pinnules in living specimens of *S. inerme. I have observed no pinnules in 

 S. pdlucidum (Fig. 20), S. bogorovi, S. hyperboreum, S. variabile, S. tenue, 

 S. meridiale, S. norvegicum and S. minutum. 



The tentacular crown is very long in all Pogonophora. In Polybrachia 

 annulata, for instance (Fig. 7), the crown reaches 16 mm while the front end 

 of the body is only c. 0-5 mm across. The single tentacle of Siboglinum 

 (Figs. 3, 20) is relatively even longer and able to twist up into a tight cork- 

 screw (Fig. 56) (Caullery, 1944). 



