SIBOGLINUM SUBLIGATUM 281 



to ten times as long as broad. The boundary between the protosoma and 

 mesosoma is a simple annular groove lying a little in front of the bridle. The 

 comparatively short protosoma is cylindrical. The large conical cephalic lobe 

 is pointed at the tip and is directed straight forward. The point of attachment 

 of the tentacle lies a little nearer to the hind edge of the protosoma than to 

 the tip of the cephalic lobe (Fig. АА133Д D). 



The tentacle is moderately long but rather thick, often attaining a diameter 

 of 0-1 mm, though it tapers gradually towards the end. In all individuals it 

 seems to be in a state of contraction, though its surface almost always lacks 

 wrinkles. There are no pinnules. It is most characteristic that the tentacle 

 shows no tendency to coil into a corkscrew, such as we find in so many 

 species of Siboglinum. 



The bridle is found at the front end of the mesosoma. Its delicate keels, 

 lying on narrow epidermal ridges, are simple and colourless. There is a 

 broad gap between their dorsal ends, and the ventral ends gradually taper 

 but do not meet (Fig. АА133Д B). On the ventral side of the mesosoma, 

 between the ridges of the bridle and a little in front of them, the integument 

 forms two small triangular glandular areas with clearly marked borders 

 (Fig. АА133у4). Behind the bridle and adjacent to it on the lateral surfaces 

 of the body, begin the pair of broad longitudinal glandular ribbons of the 

 mesosoma, which are whitish in reflected and opaque in transmitted light. 

 They then turn onto the ventral side of the body and extend backwards 

 parallel to one another to the hind end of the mesosoma (Fig. AA133). 

 When they are sufficiently large they give the impression of consisting of 

 large flat glandular epidermal cells filled with a small-grained secretion. 

 Sometimes the nucleus may be observed in these cells as a small light area. 

 The dorsal surface of the mesosoma has no special features. 



The boundary between the mesosoma and metasoma is marked by a 

 characteristic wasp-waist, from which the species is named (Lat. subligatus — 

 bound or laced in). The mesosoma narrows suddenly on the ventral side, 

 forming a sort of short stalk which joins it to the metasoma and which is 

 particularly well seen when the animal is viewed from the side (Fig. AA133Z)). 



Fig. E133. Siboglinum vekronis. A -entire tube containing animal, X 22; В - part of tube showing 

 paired dark alternating with pale bands, X 89 ; C- part of tentacle from near base, showing pinnules 

 curled toward midventrum, X 228; D- another part of tentacle from another invidual, with pinnules 

 nearer tip of tentacle, X 228; £-animal with tube partly dissected away, in left lateral view, X 25-5; 

 F- anterior regions, including protosoma, mesosoma with bridle and part of glandular metasoma, in 

 left lateral view, X 83 ; G - a toothed platelet from the annular zone, seen from the denticled side, 

 X 5,720, Я -one seen from the opposite side, and J'-from the top showing arcuate arrangement 

 of denticles. (After Hartman 1961.) 



