SIBOGLINUM PLUMOSUM 



199 



In the front part of the trunk the strongly compressed and hence laterally 

 elongated papillae (Fig. 103у4), which closely limit the deep narrow ventral 

 sulcus, are irregularly shaped and show incomplete metamerism. Inside 

 each one may be seen a rounded pyriform gland, but there are no cuticular 

 plaques. In the hind portion of the metameric part, the papillae become 

 rounded (Fig. 103C) and here, on the surface of each papilla, may be noticed 

 the aperture of a pyriform gland forming a pore in the centre of a rosette. 

 The broad dorsal ciliated band (Fig. 1035) begins immediately at the front 

 edge of the trunk and stretches the whole length of the metameric region. 



The parts of the body have the following dimensions : the length of the 

 fore-part, 2-5 mm; its breadth, 0-5 mm; the length of the tentacle, 19-5 mm. 



The dirty-brown thick-walled firm tube is 

 rather rough, with an uneven surface, and con- 

 sists of narrow, dark yellow ochre rings, sepa- 

 rated by very narrow clear belts (Fig. 104). In 

 the middle of each ring are irregular clear 

 blotches, strung out in transverse rows, seem- 

 ing to indicate that the rings are formed by 

 the fusion of neighbouring pairs of rings. The 

 length of the tube, torn off at both ends, is 

 27-5 cm, and its diameter 0-6 mm. 



5. plumosum belongs to the group of species 

 characterized by possessing a bipectinate ten- 

 tacle and a pretentacular groove {S. ekmani, S. 

 pinnulatum and S. fedotovi). To all appear- 

 ances it is particularly closely related to S. 

 fedotovi, with which it is linked by the struc- 

 ture of the tube also: in both species this 

 shows a tendency for the rings to form doub- 

 lets. S. plumosum is distinguished from all Fig. 104. Siboglinum plumo- 



these species by the absence of any sign of а w«: Tube, a -remains of the 



. _ . _ . , , r . . . ., spaces between fusing rings, 



ventral fusion of the keels of the bridle. (After ivanov 1957a ) 



Fig. 103. Siboglinum plumosum: A - front end of a female from the ventral side; В - the same from 

 the dorsal side ; С - middle section of the metameric part of the preannular region in ventral view. 

 с - cephalic lobe; ci - glandular girdle; со - dorsal ciliated band;/- keel of the bridle; ms - meso- 

 soma; pa - papillae; pgt - pores of pyriform glands; pi - pinnules; ps - protosoma; s 1 - groove 

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

 s 3 - pretentacular groove; sv - ventral sulcus; / - tentacle. (After Ivanov, 1957a.) 



