116 



PART I. GENERAL ACCOUNT 



-U 



is no trace of any other tentacle rudiment. A broad coelomic canal runs 

 inside the tentacle lined with peritoneum, but there is no blood vessel as yet, 

 nor any trace of pinnules. Little doubt thus remains that the single tentacle 

 of the adult Siboglinum is formed on the right side of the body and that it is 

 homologous with the first-formed tentacle on the right in Oligobrachia. 

 There is still a broad ciliated belt in front of the tentacle. 



The mesosoma is very long. There is still no bridle but a girdle of thickened 

 epithelium, glandular in nature, is developed in the anterior part of the seg- 

 ment. The short metasoma is cut off from the mesosoma by a wasp-waist and 

 an internal diaphragm. The posterior half of the metasoma is much thicker 

 than the rest and bears a narrow ciliated band which is more strongly de- 

 veloped on the dorsal side. Eight bristles, grouped into two dorsal and two 

 ventral pairs, lie in front of this belt (Fig. 82). 



The gut rudiment, which lacks any perceptible lumen, takes up a con- 

 siderable part of the body. There is neither oral nor anal aperture. The 

 histological structure of the bristle sacs is similar to 

 that of the embryo of Oligobrachia, but the internal 

 ends of the bristles have fine muscle fibres attached, 

 acting as protractor muscles and inserted in the body 

 wall (Fig. SIC). 



Thus during the course of embryological develop- 

 ment in the Pogonophora, the posterior segment — the 

 metasoma — is the first to be divided off and only after 

 this has taken place are the protosoma and mesosoma 

 separated. The metasoma nevertheless lags behind the 

 anterior two segments in growth and retains an em- 

 bryonic structure. Its growth in size and length and its 

 differentiation must take place when the young animal, 

 released from the maternal tube, is already leading an 

 independent existence and is perhaps preparing its 

 own tube. The late differentiation of the metasoma 

 is manifested in particular in the tardy appearance of 

 the posterior ciliated girdle. It is known that in 



Fig. 82. Late embryo of Siboglinum caulleryi, seen from the ventral side. 

 с - cephalic lobe ; ca - anterior ciliary girdle ; с h - bundle of provisional 

 bristles; cp - posterior ciliary girdle; gt - pyriform glands seen by 

 transparency; ms - mesosoma; mts - metasoma; ps - protosoma ; 

 s 1 - groove between the protosoma and the mesosoma; s 2 - groove 

 between the mesosoma and the metasoma ; td - right anterior (and 

 only) tentacle. (After Ivanov, 1957b.) 



— s' 



— ms 



