10. EMBRYOLOGICAL DEVELOPMENT 



115 



metasoma which still remains very short and completely lacks any trace of 

 the definitive organs, retaining its embryonic character. 



The body by now is 1.9 mm long. The protosoma already possesses almost 

 the same proportions as in the adult and it is furnished with a rather long 

 tentacle which is attached somewhat to right of the mid-ventral line. There 



Fig. 81. Transverse sections through the metasoma of late embryos: A- Bristle sacs and young 

 bristles in Oligobrachia dogieli; В -section through the metasoma of the same species, with well- 

 formed bristles (lower magnification) ; С - bristle sac of Siboglinum caulleryi. 

 ch - bristles ; ch 1 - young bristle which has not yet pierced through the ectoderm ; сое III - coelom of 

 metasoma; ec - ectoderm; en - endoderm; mc - circular muscle fibre; pr - muscle fibre -bristle 

 protractor; sac - brisde sac. (After Ivanov, 1957b.) 



