18 



PART I. GENERAL ACCOUNT 



uv 



The preannular region of the trunk is distinguished also by the possession 

 of a broad dorsal ciliated band of integument (Caullery, 1944; Johansson, 

 1939), which begins at the front end of the metasoma and stretches usually 

 to the end of the metameric part (Figs. 4, 5, 9, 15). In Polybrachia annulata 

 there is another similar dorsal ciliated band in the zone of thickened papillae. 



Two or three girdles [or rarely four or five] 

 of toothed platelets lie in the middle section 

 of the trunk. Most frequently there are two 

 girdles placed close together one behind the 

 other on muscular ridges. They are usually 

 interrupted ventrally (Figs. 2, 12, 16), and 

 in many species either or both girdles are 

 broken also on the dorsal side. Between the 

 girdles, or in front or behind them as well, are 

 usually several well-developed mid-ventral 

 papillae, furnished almost always with adhes- 

 ive plaques. In Siboglinum pinnulatum and S. 

 bogorovi there is a considerable gap between 

 the two girdles. A whole series of species of 

 Siboglinum is characterized by the possession 

 of three girdles, of which the first two lie close 

 together and the third is situated quite a long 

 way behind them (e.g. S. robustum, S. buccelli- 

 ferum etc). [Siboglinoi'des dibrachia is unusual 

 in possessing four or five girdles disposed in 

 two groups with a considerable space between 

 the two.] In some species of Siboglinum the 

 number of girdles is subject to individual 

 variation. Each girdle (or annulus) consists 

 of two or several rows of very small (never 

 more than 30/x long) oval chitinous* platelets 

 Figs. 12, 13), [sometimes called uncini by 

 some authors by analogy with the uncini of 

 polychaetes]. Their surface is covered with 

 tiny teeth (Caullery, 1944). Each platelet shows 



ъ 



о 



^J 



(3 



Fig. 11. Part of the metasoma of a female Polybrachia annulata in the region of the thickened papillae. 



From the ventral side. 



p - adhesive plaques; pa - papillae of the thickened zone; vv - ventral blood vessel seen by 



transparency. 



* Chitin is unproven — D.B.C. 



