2. EXTERNAL MORPHOLOGY AND THE GENERAL BODY PLAN 



11 



The divisions of the body 



The body of the Pogonophora consists of three segments greatly differing 

 in their length — the protosoma, mesosoma and metasoma (Figs. 2-5, 7, 8, 

 15, 16, 20), which correspond to the proboscis (or cephalic shield or preoral 

 lobe), the collar and the trunk respectively of the Hemichordata (Ivanov, 1952, 

 1955a). The protosoma is furnished with a protuberant antero-dorsal cephalic 

 lobe (Figs. 3, 5, 8, 15) and with long tentacles. The mesosoma and metasoma 

 lack appendages. External distinctions between the segments are by no means 

 present in all the Pogonophora. In the more advanced forms the protosoma 

 fuses with the mesosoma to form a combined fore-part of the body. [The term 

 fore-part of the body will be used to refer to protosoma + mesosoma.] 

 Between the mesosoma and metasoma, on the other hand, the boundary is 

 always very clearly defined. 



Apart from this true (or primary) segmentation there is a secondary divi- 

 sion of the body into regions. Thus the metasoma in all Pogonophora is 

 divided into two sharply distinguished regions (Figs. 2, 8, 16). In some forms, 

 moreover, the protosoma, and in others the mesosoma, may be secondarily 

 divided by one or more annular grooves. 



The protosoma is the first and shortest seg- 

 ment of the body. Usually it is much shorter 

 than the mesosoma and rarely is it noticeably 

 longer than broad. For the most part it is more 

 or less cylindrical, but in S. minutum it makes a 

 pair of lateral swellings (Fig. 4). 



In the majority of Pogonophora (all the 

 Athecanephria, Heptabrachia, Poly bra chia etc) 

 the protosoma is divided from the mesosoma by 

 an annular groove, complete or interrupted 

 dorsally or ventrally (Figs. 3, 4, 8, 15). Some- 

 times this groove makes a sharp bend on the 

 dorsal side, arching backwards (Oligobrachia, 

 Fig. 15; Siboglinum caulleryi, Fig. 3) or for- 

 wards (Heptabrachia subtilis). In Zenkevitchiana, 

 LamellisabeUa and Spirobrachia the protosoma 

 is not externally distinguishable (Figs. 2, 5, 16), 

 but is fused with the mesosoma to form part of 

 an integral fore-part of the body (Ivanov, 

 1957a). 



Fig. 4. Front part of the body of 

 Siboglinum minutum from the 

 dorsal side. 



с - cephalic lobe; со - dorsal 

 ciliated band; f- bridle (frenu- 

 lum); ms - mesosoma; mts - 

 metasoma ; ps - protosoma ; t - 

 tentacle. 



