74 



PART I. GENERAL ACCOUNT 



Fig. 50. Striated muscle fibres from the wall of the ventral blood vessel of Lamellhabella zachsi. 



A - tangential section; В - transverse section. 



fin - contractile fibre; my - myocyte; sa - blood in the ventral vessel. (After Ivanov, 1960a.) 



which appears to be a continuation of the intima of the ventral vessel. When 

 the heart is contracted the intima forms numerous longitudinal folds. A 

 rather spongy connective tissue layer is developed around it, consisting of 

 irregular and often stellate cells. Outside this again we see the first signs of 

 the muscular mantle, consisting of a few circular layers interspersed with 

 streaks of diagonal (or possibly spiral) muscle fibres. In different layers these 

 run in opposite senses, forming right-hand and left-hand spirals around the 

 heart (Fig. 51). Finally, on the outside are the longitudinal fibres. So far as 

 we know the muscle fibres of the heart are smooth, unstriated fibres [unlike 

 those of the blood vessels]. The heart is not sharply delimited on the outside 

 from the surrounding connective tissue. 



When the heart is full of blood its cavity is grossly distended and the 

 intima loses its folds and, indeed, becomes highly stretched. The whole 

 structure of the heart and its location at the base of the tentacular crown 

 suggest that it is a contractile organ whose function it is to drive the blood 

 through the system of tentacular vessels. [Observations upon living Sibo- 

 glinum spp., in which the red blood may be seen to circulate and the heart to 

 contract rhythmically, tend to confirm this suggestion — D.B.C.] 



The pericardium 



In the Athecanephria a small epithelial sac lies close to the heart on the 

 dorsal side. It is completely closed, and half moon-shaped in transverse 



