8 PART I. GENERAL ACCOUNT 



Jagersten (1956), however, following Caullery (1944) and Dawydoff(1948), 

 takes the opposite view of the orientation of the body. In his opinion, the 

 ciliated band on the trunk represents the remnant of the primitive organ of 

 locomotion, which was developed on the ventral side in crawling ancestors. 

 Furthermore, Jagersten assumes that the tentacles were already developed in 

 these creeping ancestors of the Pogonophora on the side away from the 

 substratum, i.e. on the dorsal side. 



These arguments possess, however, the character of unproven speculative 

 supposition, and strong objections may be raised against each of them. Indeed, 

 there is no evidence whatever that the ciliated band ever served a propulsive 

 function or that it was ever part of a ventral locomotory organ. Rather there 

 can be no doubt that it represents the remnant of a general ciliated epithelium. 

 A crawling type of ancestral form is thus clear supposition. On the contrary, 

 the development of tentacles was, in all likelihood, bound up with a sedentary 

 way of life. 



It is perhaps more reasonable to consider that since the Pogonophora, in 

 common with the lower Deuterostomia, display a trisegmental structure 

 (p. 11), their median nerve cord, always well developed for the whole length 

 of all three segments of the body, must be dorsal. 



External anatomy 



The vermiform and, for the most part, threadlike body of the Pogonophora 

 has the characteristic shape of a sedentary animal, bearing on the anterior end 

 a crown of tentacles like that of sedentary polychaetes, phoronids, bryozoans 

 or Pterobranchia. The superficial resemblance to sedentary polychaetes is 

 often enhanced by the presence on the trunk of more or less metameric 

 paired adhesive papillae with cuticular plaques, creating the impression of 

 polymeric segmentation looking like that of annelids. 



Size and colouration 

 The length of the body varies between wide limits. Amongst the smallest 

 species we may take as typical Siboglinum pellucidum and S. minutum, in 

 which the body is no more than 5-5 cm long and only 0- 1 mm (100/x) broad, 

 [while S. veleronis is only 0-065 mm (65/м) across.] Amongst the largest 

 species are Lamellisabella zachsi (Fig. 2) and Spirobrachia grandis. The length 

 of the latter species, including the tentacular crown, probably exceeds 25 cm, 

 while the trunk may reach 2-5 mm in diameter. The more slender Zenke- 

 vitchiana longissima, however, with a breadth of no more than 0-8 mm, may 

 reach 36 cm long, [while Galathealinum brachiosum, of which we do not 



