62 



PART I. GENERAL ACCOUNT 



cut— 



Fig. 43. Transverse section of the body wall of 

 Oligobrachia dogieli in the region of the protosoma. 

 сер - epidermal cell ; en - nerve cell ; cut - cuticle ; 

 fn - layer of nerve fibres ; mb - basement membrane ; 

 me - circular muscle fibres ; ml - longitudinal 

 muscle fibres ; my - muscle cell-body (myocyte) ; 

 vl- lateral cephalic vessel. (After Ivanov, 1958b.) 



Fig. 42. Diagram of the structure of the front end of the pogonophoran body according to 



Jagersten. 



cd - nephridium; сое I - first coelom; сое II - second coelom; сое III - third coelom; cor - heart; 



gp - genital aperture ; ms - mesosoma ; mts - metasoma ; na - cervical nerve ring ; nd - nerve trunk ; 



ps - protosoma; vd - dorsal blood vessel (called ventral by Jagersten); vv- ventral vessel (called 



dorsal by Jagersten). (After Jagersten, 1956.) 



Jagersten considered the cavity at the base of the tentacle in Siboglinum 

 ekmani to be merely the central part of the protocoele, thinking that three 

 blind coelomic pockets run forward from it into the cephalic lobe to house 

 the three cephalic blood vessels (Fig. 42). My own studies of the protosoma, 

 however, in S. caulleryi, Oligobrachia dogieli and a whole series of the 

 Thecanephria, do not corroborate this view. The cavity which gives rise to 

 the coelomic canals has clearly defined limits and never extends into the 

 cephalic lobe. The cells between the cephalic blood vessels and the body wall, 

 which might at first glance be regarded as peritoneal elements, are apparently 

 myocytes of the longitudinal musculature of the dermal muscle sac (Fig. 43). 

 Jagersten is, however, right to the extent that derivatives of the wall of the 

 protocoele are present in the cephalic lobe. 



