14. SYSTEMATIC POSITION OF THE POGONOPHORA 133 



teeth, as is well known, are developed in very different animals where they 

 are necessary for steadfast attachment to the substratum. On the contrary, 

 it would have been surprising if a tubicolous animal, which lacks any organic 

 connexion with the tube and which withdraws its front end in a flash when 

 disturbed, did not possess any special provision for securely attaching the 

 hind end of the body deep in the tube. 



The profound differences in their mode of formation underline the inde- 

 pendent origins of the toothed bristles of pogonophores and the uncini of 

 polychaetes. The former, as we have seen (p. 38), are each secreted inside a 

 single cell, while the latter represent cuticular material secreted outside the 

 cells. There can thus be no homology between the formation of the two. 



The parapodia-like tufts of bristles on the metasoma, possessed by the 

 nearly fully formed embryos of Siboglinum and Oligobrachia, might also 

 suggest an affinity with the annelids. These structures, however, do not show 

 any similarity of detail with the parapodia of annelids. Indeed, in Pogono- 

 phora, each provisional bristle is produced inside a multicellular or syncytial 

 sac, all of whose cells take an equal part in the operation : there is no one cell 

 amongst them specialized for this work such as we find in annelids. Hence the 

 bristle sacs of embryonic Pogonophora and the parapodia of Polychaeta can 

 be regarded as no more than analogous structures. To be sure, despite the 

 marked distinction, it is impossible not to remark on the great similarity 

 between them. This is hardly surprising, for in a number of groups of the 

 animal kingdom there are frequently close analogies in many structural 

 details. 



Thus, for example, some Brachiopoda, especially Lingulidae, Chlidono- 

 phora chuni and others, possess numerous setae along the edge of the folds of 

 the mantle. These setae or bristles are enclosed in deep cutaneous invagina- 

 tions and each one is formed within a single giant cell which clasps its base 

 (Blochman, 1900). The whole structure is very similar to the bristle sac of the 

 parapodia. The likeness is increased by the similar finely fibrous structure of 

 the setae of Brachiopoda and Polychaeta. Moreover, the larvae of some 

 articulate brachiopods (Argiope, Terebratulina, etc) are furnished with pro- 

 visional bristles or setae. Kovalevskio (1874) pointed out that these are 

 astonishingly like the parapodia of larval polychaetes. Nevertheless, despite 

 the possession of these parapodia-like structures in both larva and adult, no 

 one would pretend that the Articulata have anything in common with the 

 Annelida. And the analogous bristles of Pogonophora give just as little 

 foundation for any phylogenetic connexion with the polychaetes. 



The genital system of Pogonophora seems at first glance to have more in 



F 



