2. EXTERNAL MORPHOLOGY AND THE GENERAL BODY PLAN 25 



non-metameric part bears never more than 50 papillae and in some 

 individuals far fewer (Fig. 16). 



It is also of interest that the cuticular plaques on the anterior metameric 

 papillae are more highly differentiated than on the papillae farther back; this 

 is especially striking in the higher more specialized species. Thus in Lamel- 

 lisabella they are distinguished by their complex shape, and function as a 

 rather efficient catch mechanism, while the plaques on the scattered papillae 

 of the middle part of the trunk are still very primitive (Fig. 34). In the higher 

 species, moreover, special pulvilli or cushions are developed on the meta- 

 meric papillae which bear the cuticular plaques. These are found in Bir- 

 steinia among the Athecanephria and Lamellisabella and Spirobrachia among 

 the Thecanephria. In this way the localization and differentiation of a 

 multiplicity of organs proceed parallel with the superposition of order. 



Everything mentioned above tends to show that the development of 

 secondary metamerism (or pseudometamerism) in the Pogonophora has 

 come about by the imposition of order upon an originally disorderly array of 

 a great many similar organs. Evidently in the process of evolution the meta- 

 merization of the papillae of the preannular region of the trunk began at the 

 anterior end. Then it spread backwards, engrossing an ever-increasing 

 number of these organs. In the first stage of evolution all the adhesive 

 papillae, to all appearances, are completely higgledy-piggledy (Ivanov, 1957a, 

 1959f). 



The tentacles 



The tentacles of the Pogonophora should perhaps rather be called arms or 

 brachia, since they are extensions of the body enclosing a coelomic canal. 

 They are attached at the front end of the protosoma, ventral to the base of 

 the cephalic lobe, and in all species, except in the unitentaculate Siboglinum, 

 they form a tentacular crown. 



The number of tentacles fluctuates between wide limits: Siboglinum 

 (Fig. 3) has only one tentacle; [Siboglinoi'des, Sclerolinum and Nereilinum, 

 two;] Diplobrachia, two to four; Oligobrachia (Fig. 15), six to nine; Hepta- 

 brachia, five to thirteen; Cyclobrachia, nine; Birsteinia, 12; Zenkevitchiana, 

 14; Poly brachia (Fig. 7) four to 70 or more in different species; Lamellisabella 

 zachsi (Fig. 2), 28 to 31 ; Galathealinum bruuni, more than 105; Spirobrachia 

 grandis as many as 223; [and Galathealinum brachiosum, 268]. 



The most primitive species possess few tentacles [6-8]: e.g. Oligobrachia 

 and Heptabrachia. The single tentacle of Siboglinum appears to be the result 

 of secondary oligomerization, as is clear from the peculiarities of its innerv- 

 ation (Ivanov, 1956b, 1958b). In pogonophores each tentacle is innervated 



