388 PART II. SYSTEMATIC ACCOUNT 



4. Genus Zenkevitchiana Ivanov 1957 



Ivanov, 1957a: 439, 448, 470-5, 483, 490, 492, 493, 496, 497; Abrikosov, 1957: 238; 

 Dajoz, 1958: 236; Ivanov, 1960a: 1528, 1531, 1539, 1558, 1620; Ivanov, 1960c: 6, 11, 15, 

 24, 37, 92, 94, 106, 108, 114, 199, 215, 226-7. 



The protosoma is continuous with the mesosoma and the cephalic lobe is 

 not separated from the protosoma. The tentacular crown consists of a 

 comparatively small number of tentacles. The front part of the trunk lacks 

 cuticular plaques and the adhesive papillae are glandular. The tube is seg- 

 mented, without funnel-shaped frills to the segments, and the segments in 

 the front part are white and flexible. The inclusion of this genus among the 

 Polybrachiidae is at first glance contradicted by the merging of the protosoma 

 and the mesosoma into a single united fore-part of the body, a feature not 

 found in other genera of Polybrachiidae, but, on the contrary, characteristic 

 of the Lamellisabellidae and Spirobrachiidae. In the present case, however, 

 a more important indication would seem to be the possession of free 

 tentacles, while the fusion of the two anterior segments appears increasingly 

 in the more highly evolved species of Thecanephria and must have arisen 

 independently in the Polybrachiidae, Lamellisabella and Spirobrachia. The 

 absence in Zenkevitchiana of any cuticular plaques in the fore-part of the 

 trunk may be a result of their reduction, and thus cannot be considered to 

 hinder the ascription of this genus to the Polybrachiidae (Ivanov, 1957л). 



Type of the genus : Zenkevitchiana longissima Ivanov. 



1. Zenkevitchiana longissima Ivanov (Figs. 9, 34, 153-5) 



Ivanov, 1957a: 439, 448, 470-5, 483, 490, 492, 493, 496, 497, Figs. 41-45, 59; Abrikosov, 

 1957: 238; Brunei and Carlisle, 1958: 1689; Ivanov, 1960a: 1538, 1542, 1606, 1607, 1613, 

 Figs. 1391, 1415; Ivanov, 1960c: 6, 10, 22, 91, 94, 97, 108, 151, 227-30, 266, Figs. 9, 34, 

 87, 153-5. 



This species is characteristic of the Kuril Trench, where in places more 

 than 9000 m deep it occurs in enormous numbers. The fore-part of the body 

 is cylindrical, with a length to breadth ratio of 6 : 1 (Fig. 153A, B). The 

 region of the protosoma is very small, strongly compressed and not divided 

 from the mesosoma. In the mesosomal region masses of small round saccu- 

 lar^ glands may often be seen through the body wall. The small cephalic lobe 

 is somewhat flattened and rounded at the tip. The 14 very long, unfused 

 tentacles are arranged in a complete ring and each bears an apparently single 

 row of very small pinnules. The very delicate, pale keels of the bridle, which 

 lies in the front half of the fore-part, almost meet dorsally and fuse together 



