2. EXTERNAL MORPHOLOGY AND THE GENERAL BODY PLAN 



19 



Fig. 13 



Fig. 12. The girdles of Siboglinum caulkryi in a distended condition. 



an - girdles; pa - median papilla behind the girdles; pan - the beginning of the postannular region 



of the metasoma. 



Fig. 13. Toothed bristles of Polybrachia annulata. 



A - a bristle from the side ; В - toothed platelet of bristle. 



externally only its head of cutaneous bristles or teeth (Fig. 13, see also Fig. 31). 

 The teeth, arranged in rows on the surface, form two groups — an anterior 

 group, which usually consists of comparatively few small and weakly developed 

 thorns or teeth, regularly pointing backwards, and a posterior group, con- 

 sisting of numerous longer teeth, pointing forwards. In different species of 

 Siboglinum all degrees of reduction of the anterior teeth may be observed. 

 Thus in S. buccelliferum and S. pinnulatum 

 the two groups are almost equally developed, 

 in the majority of species the anterior group 

 is more or less rudimentary, and in S. 

 minutum and S. vinculatum it has completely 

 disappeared. The girdles, without any doubt, 

 serve to fix the middle part of the body 

 firmly in the tube, when, as must happen, 

 the animal is disturbed and instantly with- 

 draws the front end of the body and the 

 tentacles into the tube by contraction of the 

 longitudinal musculature. 



Behind the girdles there is usually a notice- 

 able narrowing or waist, after which the 

 postannular section of the trunk begins. This 

 latter is very long and more or less cylin- 

 drical, tapering towards the hind end and 

 rather different in the two orders. In the 



Fig. 14. Dorsal shield of the 

 postannular region of the meta- 

 soma of Siboglinum caulkryi. Seen 

 from the side. 



pgt - pore of pyriform gland ; 

 sc - dorsal shield. (After Ivanov, 

 1960a.) 



