138 PART I. GENERAL ACCOUNT 



These features, however, are shared by all the lower Deuterostomia and 

 afford no grounds for including the Pogonophora within the phylum 

 Hemichordata. We must reach the opposite conclusion if we consider the 

 following list of the features of organization which are peculiar to the 

 Pogonophora : 



1. The development of tentacles on the protosoma. We do not meet this 

 peculiarity in other Deuterostomia, where tentacular outgrowths — such as 

 the lophophores of Pterobranchia, the ambulacral outgrowths of Echino- 

 dermata or the oral cirri of Acraniata — are always attached to the second 

 segment. 



2. The great development of the coelomoducts in the trunk segment. 

 Trunk coelomoducts are only otherwise developed in Vertebrata, unless we 

 include the problematical atrio-coelomic funnels of Branchiostoma and do not 

 accept the conclusion of Felix and Biihler (1906) that the gonads of Acraniata 

 are homologous with coelomoducts. The Pogonophora are distinguished 

 from the Hemichordata, moreover, by the absence of mesosomal coelomo- 

 ducts. 



3. The development of the chief ganglionic nerve centre in the protosoma. 



4. The primitive nature of the genital system as expressed in the close 

 association of the gonad with the coelom and in the trunk coelomoduct's 

 functioning as a genital duct. This is, of course, also found in the vertebrates. 



5. The differentiation of a muscular heart on the ventral vessel. No homo- 

 logue is possessed by other Deuterostomia. 



6. The strongly cuticularized skin. 



7. The complete absence of any gut, and its function being taken over by 

 the tentacular apparatus. 



8. The possession of spermatophores. 



9. The greatly modified embryological processes, with no trace of an 

 invaginated gastrula, a blastopore, secondary mouth or pelagic larva. 



The absence of gill slits and stomochord or notochord might be no more 

 than consequences of the secondary reduction of the whole gut. 



The ability of Pogonophora to produce their own chitinous tube further 

 distinguishes them from other Deuterostomia. But Hyman (1958) correctly 

 remarked that the distribution of chitin throughout the animal kingdom can 

 hardly be regarded as a phylogenetic pointer. The only other group amongst 

 the Deuterostomia in which chitin might possibly occur is the Hemichordata. 

 The evidence is by no means clear upon the constitution of the coenoecia of 

 Rhabdopleura, Cephalodiscus and the Graptolithoidea. Rudall (1955) could 

 find no trace of chitin in the tube of Rhabdopleura. [I have found no trace of 



