14. SYSTEMATIC POSITION OF THE POGONOPHORA 139 



N-acetylglucosamine or glucosamine in hydrolysates of the coenoecia of 

 Rhabdopleura and Cephalodiscus, while the organic remains of graptolites, 

 like the tubes of Pogonophora, yielded these substances upon enzymic and 

 acid hydrolysis respectively — D.B.C.] 



Many of the features of the organization of pogonophores were un- 

 doubtedly acquired under the influence of their sedentary way of life. Under 

 this head we may include such features as the replacement of the gut by a 

 tentacular system developed on the front end of the body, performing the 

 functions of food collecting, digestion and absorption. We may also include 

 the secretion of a chitinous tube, whose possession is associated with the 

 extraordinary elongation of the body, and the adaptations which enable the 

 animal to move up and down within it — the bridle, adhesive papillae and 

 girdles of toothed bristles. The adhesive papillae were possibly primitively 

 scattered higgledy-piggledy all over the trunk. Then, in connexion with the 

 nature of their function, they become partially metamerically arranged, a 

 common adaptation to tubicolous life. 



We cannot in the long run escape the conclusion that pogonophores are 

 an independent phylum within the Deuterostomia (Ivanov, 1955c; Bekle- 

 mishev, 1944, 1957)*. 



In this connexion it is worth paying attention to the fact that secondary 

 metamerism of the trunk is fairly common amongst the Deuterostomia. It is 

 well known that in Enteropneusta the gill slits, gastro-intestinal pores, 

 branchial blood vessels, hepatic caeca and gonads are all metameric. And 

 metamerism of the Acraniata and vertebrates is also essentially confined to 

 the third of the three primitive segments. In the embryological development 

 of Branchiostoma, as is well known, there is a stage when it clearly shows the 

 ancestral trisegmental body plan (MacBride, 1914; Fedotov, 1923; Bekle- 

 mishev, 1952). The anterior (proboscis) coelom and the middle ("collar") 

 pair of coelomic cavities of the embryo play a very modest part in the organ- 

 ization of the adult Amphioxus. But the hind pair of coeloms of the embryo 

 homologous with the third pair of coelomic sacs of the dipleurula and with 

 the coeloms of the trunk of Hemichordata and Pogonophora, form almost all 

 the mesoderm of the adult animal. As the embryo develops the hind pair of 

 coeloms elongate rapidly and undergo secondary metamerization by dividing 

 into somites. This process goes on to produce the metameric musculature of 

 the adult Amphioxus and the consonant metamerism of the nervous system, 

 gonads, gill slits, blood vessels and excretory organs. 



* A. M. Obut (1957) regards the Pogonophora as a subphylum of the Hemichordata. His 

 arguments cannot, however, be taken seriously. 



