44 



PART I. GENERAL ACCOUNT 



S. caulleryi (Fig. 30) and S. minutum, and in all likelihood, in all species of 

 this genus. In Lamellisabella and Spirobrachia the longitudinal muscle layer 

 forms a pair of considerable thickenings on the ventral side of the body in 

 the front part of the metasoma. 



The longitudinal musculature consists of very long ribbon-like fibres, 

 arranged, in transverse section, perpendicular to the circular muscle layer. 

 Not infrequently the fibres form only a single layer (Fig. 33), but in the 

 larger species (Polybrachia, Lamellisabella, Spirobrachia) they are arranged 

 in many layers (Figs. 27, 29, 39, 45, and 52). On the side of the muscle layer 

 which is towards the body cavity is a continuous layer of cells with round 

 nuclei. Some of these cells appear to be the myocytes, so that the muscle 

 cells are of the nematode type, each consisting of a cytoplasmic body con- 

 taining the nucleus, jutting out into the coelom with the contractile portion 

 of the cell applied to the body wall (Johansson, 1939). It is clear that the 



muscle fibres of the cephalic lobe in Oligo- 

 brachia dogieli are also of this nematodal type 

 (Fig. 43), but in the middle region of the 

 trunk in Lamellisabella the longitudinal 

 muscle cells possess nuclei which lie inside 

 the ribbon-like fibres themselves or else 

 are closely applied to the broad face of the 

 fibres (Fig. 265). Thus, in this part of the 

 body at least, there is no continuous cell- 

 layer delimiting the longitudinal muscle layer 

 from the coelom, but on the contrary the 

 coelom may be considered devoid of peri- 

 toneal elements. 



At the level of each girdle of toothed 

 bristles a special muscular ridge is developed 

 at the expense of the longitudinal layer of the 

 dermal muscle sac (Fig. 31). Its contraction 

 thrusts the region of the girdle outwards so 

 permitting the toothed heads of the bristles 

 to hook onto the walls of the tube (Fig. 12). 

 Some of the fibres of this muscular ridge join 

 the basement membrane of the girdle to a 

 small portion of the cutaneous epithelium 

 which lies a little in front of it. It is interesting 

 that each fibre is attached to the basement 



Fig. 31. Musculature of the girdle 

 of toothed bristles of Lamellisabella 

 zachsi in longitudinal section. 

 с h - toothed bristles ; cut - cuticle ; 

 ep - epidermis; m x - muscle fibres 

 causing the girdle to protrude; 

 m 2 - muscle fibres pulling in the 

 girdle; ml - longitudinal muscle 

 layer; per - peritoneum. (After 

 Ivanov, 1960a.) 



