4. MUSCULATURE AND PAPILLAE 



43 



way the keels may be forced against the walls of the tube and hold the fore- 

 part of the body fist (Ivanov, 1958b). 



The longitudinal musculature forms a continuous layer, attaining, in large 

 species, a considerable thickness. Only in a few parts of the body does it 

 become thinner, such as mid-ventrally and mid-dorsally, along the lines of 

 attachment of the mesentery (p. 64). It is usually developed to roughly the 

 same extent along the whole length of the body, except for a slight thickening 

 in front and a gradual dwindling in the postannular region of the trunk. In 

 Siboglinum weberi, however, the major part of the longitudinal muscle fibres 

 is concentrated on the dorsal side; in the middle part of the mesosoma they 

 are divided into two dorso-lateral tracts, which a little farther back turn onto 

 the sides of the body where they become more powerful; finally, in the hind 

 part of the mesosoma the fibrous tracts become uniformly dispersed all 

 round the body (Caullery, 1944). The same is true of the muscle tracts in 



Fig. 30. Transverse section of Siboglinum caulleryi in the mesosomal region. 

 eg - gland cells of the epidermis; сое II - coelom of the mesosoma; cut - cuticle; mc - circular 

 musculature; mes - mesentery; ml - longitudinal musculature; nd - dorsal nerve trunk; 

 per - peritoneum; vd - dorsal blood vessel; vv - ventral blood vessel. (After Ivanov, 1960a.) 



