SIBOGLINIDAE 175 



conical or paddle shaped cephalic lobe is often divided from the protosoma 

 by a pretentacular groove, absent in related species some of which may 

 possess instead a post-tentacular groove behind the base of the tentacle. Not 

 infrequently, however, the protosoma is undivided. The long tentacle, which 

 is capable of twisting into a tight corkscrew spiral, usually bears one or two 

 rows of pinnules, but in some small species these are absent. 



The mesosoma, as a rule, lacks secondary rings, but in some species there 

 are one or two transverse grooves in front of the bridle. The keels of the 

 bridle may be well marked or very faint, with tiny cuticular rods laid across 

 them, and behind them is often a more or less complete glandular, epidermal 

 girdle, while sometimes long, ventral, longitudinal glandular stripes are 

 developed. 



The metameric part of the trunk carries a large number of more or less 

 paired papillae, devoid of cuticular plaques, while the papillae of the zone of 

 thickening are generally few and, for the most part, arranged in a single 

 ventral row. Their number and the extent of their development appear to be 

 good diagnostic characters. The dorsal ciliated band is well developed. The 

 number and arrangement of the girdles vary. Often there are two of them 

 close together, but they may lie separated from each other; when three 

 girdles are present the front two usually lie close together, far separated from 

 the hindmost. The platelets or toothed bristles are usually arranged along 

 the girdles in 1 or 2 rows, rarely in 3 or 4. On each platelet the teeth of the 

 anterior group (much more weakly developed than the posterior) are more 

 or less rudimentary or even absent, but in some few species they take up 

 nearly as much space as the posterior group and are almost as well-developed. 

 The postannular part of the trunk carries a series of metameric dorsal glandu- 

 lar shields, on which may be seen the pores of pyriform glands and opposite 

 each shield, on the ventral side, lies a large conical papilla, sometimes carry- 

 ing at its tip a slight thickening of the cuticle or even a cuticular plaque. 



The spermatophores are spindle-shaped. 



The tube is ringed, with simple or anastomosing rings, and in a number of 

 species it is segmented. 



Type species of the genus: Siboglinum weberi Caullery, 1914. 



At the present time the genus Siboglinum contains 38 known species. This 

 is a very extensive genus, whose various species are wide-spread throughout 

 all the oceans of the world. 



