7. THE BLOOD-VASCULAR SYSTEM 



75 



mc 



4 C<. 



mt ~Щ<^ =~ — — 



Wtj^ ^sSZ m ^ 



Fig. 51. Transverse section of the heart of Spirobrachia grandis. 



cd - coelomoduct ; cv - corpus cardiacum in the dorsal vessel ; int - intima of the vessel ; 

 mc- circular muscle fibres; ml - longitudinal muscle fibres; tc - connective tissue; vd - dorsal 

 blood vessel. (After Ivanov, 1960a.) 



section (Fig. 52). It was discovered in Oligobrachia, Birsteinia and Sibo- 

 glinum and named the pericardium (Ivanov, 1955a). The nature of this organ 

 remains uncertain. It is not known how far it may be homologized with the 

 cardio-pericardial sac of the Hemichordata. There is no evidence as yet 

 about the ontogenetical origin of this sac, but we cannot exclude the possi- 

 bility that it develops during organogenesis by separation of part of the 

 protocoele. The functional significance of the pericardium, on the other hand, 

 is fairly clear. It plays the role of a pouch or cushion making the expansion 

 of the heart easier, and so, as regards its physiology, it is sharply distinguished 

 from the cardio-pericardium of the Enteropneusta. 



The Thecanephria lack a pericardium, and it is possible that its role is 

 taken over by the renal sacs, which lie above the hind part of the heart. 



