SIBOGLINUM TENUE 239 



and of S. bogorovi were found a number of delicate tubes readily distinguish- 

 able by their double rings. Some of them contained animals, obviously 

 belonging to a different species, here named S. tenue. This species is 

 distinguished by the comparatively short, cylindrical fore-part of the body 

 which is only five times as long as broad. The conical cephalic lobe is very 

 rich in gland cells. The stout tentacle, attached a little to the right of the 

 mid-ventral line, lacks pinnules, and behind its base runs a distinct transverse 

 groove. The bridle lies about one-third of the way back along the mesosoma. 

 Its keels, as seen under the microscope, are rather distinctive, each consisting 

 of a rather broad, just perceptibly brown strip of cuticular material, crossed 

 by short dark delicate transverse lines; and along the length of each strip 

 runs a very dark slender cuticular thread, standing up slightly above the 

 surface (Fig. \2\A). The cutaneous ridges of the bridle are developed only 

 on the dorsal side, and behind the bridle lies a pair of lateral patches of 

 epidermal gland cells, forming a rudimentary glandular girdle. From the 

 sides of the body these extend chiefly onto the ventral surface, where they 

 almost meet in the mid-line, and hardly run onto the dorsal surface at all. 

 The hind edge of the mesosoma is somewhat oblique, with the ventral side 

 a trifle in advance of the dorsal (Fig. 121C). 



The first part of the trunk is noticeably narrower than the mesosoma and 

 the rather narrow dorsal ciliated band begins at its front edge. Each of the 

 comparatively large, rounded adhesive papillae contains a large pyriform 

 gland (Fig. 121C). It has not proved possible to investigate the middle part 

 of the trunk because of the difficulty of dissecting it out of the very fine tube. 

 S. tenue possesses three girdles, of which the first two, lying close to each 

 other, are at a considerable distance away from the third, which is situated at 

 the beginning of the postannular region (Fig. 12 ID). In this regard S. tenue, 

 is comparable with S. ekmani, S. fedotovi, S. hyperboreum, S. buccelliferum, 

 S. pinnulatum and S. robustum. The girdles lie on pronounced ridges, which 

 in turn lie on muscular folds of the trunk. The first and last girdles are in- 

 terrupted ventrally and the second dorsally (Fig. 121/)), and the toothed 

 platelets in each girdle are arranged in a single row (Fig. 121Z), E). They are 

 comparatively few, almost colourless and elongated, without any constriction 

 in the middle, and bear two well-developed groups of teeth. The anterior 

 teeth, pointing backwards, are rather numerous and comparatively well 

 developed (Fig. \2\F). The platelets are between 8 and 10/u, long. The long 

 delicate postannular section of the trunk carries a series of regularly spaced, 

 metameric dorsal glandular shields, and, lying on the ventral side opposite 

 each shield, papillae, which are devoid of any cuticular thickening at the tip. 



