76 



PART I. GENERAL ACCOUNT 



0.1mm 



cor 



Fig. 52. Transverse section of Oligobrachia dogieli at the level of the heart and of the pericardium. 

 eg - gland cell of epidermis ; сое II - coelom of mesosoma ; cor - heart ; cut - cuticle ; ep - epidermis ; 

 me - circular muscle layer; mdv - dorso-ventral bundle of muscle fibres; ml - longitudinal muscle 

 layer; per - peritoneum; pn - dorsal nerve plate - the origin of the dorsal nerve trunk; pr - peri- 

 cardial sac; vd - dorsal blood vessel; vl - hind part of the lateral cephalic vessel. (After Ivanov, 

 1960a.) 



The corpus cardiacum 



With the exception of Siboglinum, in all the Pogonophora which have been 

 investigated there is a very odd cellular formation within the ventral vessel. 

 This was described by Johansson (1939) as mesenchymatous tissue, but it 

 would be better named the corpus cardiacum (Ivanov, 1956b, 1960a). 



This long uninterrupted tract, consisting of large clearly vacuolated cells, 

 is attached to the dorsal wall of the ventral vessel. It is never present in the 

 mesosoma. It is separated from the lumen of the vessel by a structureless 

 bounding membrane which merges into the intima of the vessel, although the 

 latter is considerably thicker. The cells of the corpus cardiacum are piled 

 together in disorder. They are shaped like irregular polygons and are usually 

 pressed close together (Figs. 45, 46, 61, 63, 64, 68, 69). The cytoplasm is 

 densely supplied with small granules of glycogen. Thus at least one of the 



