THE LIFE-HISTORY OF NUCULA DBLPHINODONTA. 367 



pericardium (figs. 40 and 41). The pericardial space is not 

 smooth as it has been necessary to represent it in the figures. 

 Mesoderm cells project into it from the surrounding tissue^ 

 and others lie comparatively free within it. As yet it lies 

 almost wholly dorsal to the intestine^ but just before the 

 heart is formed it is extended beneath the intestine, and 

 begins to have a rather definite epithelial lining'. The epi- 

 thelial lining seems to be formed by the change in shape and 

 position of cells in the immediate vicinity. I find no indica- 

 tion that the pericardium originates as a pair of pouches, as 

 has been described by Ziegler for Cyclas cornea (20). 



Vascular System. 



Small connected cavities are present throughout the body 

 from an early time, but a true vascular system, with a heart 

 and anything like a definite circulation, is not to be distin- 

 guished until much later, and a closed system of vessels with 

 capillaries is never present. 



The heart is formed about the time that the gill becomes 

 well divided into two lobes, or just before the third lobe is 

 formed. It seems to be formed by the hollowing out of a 

 strand of mesoderm that stretches across the pericardial 

 cavity. I have seen nothing that would indicate that the 

 heart has a double origin, as Ziegler has described for 

 Cyclas cornea (20). Mesoderm cells in the pericardial 

 cavity and along its walls arrange themselves to form a 

 strand that becomes hollow and begins to pulsate. From the 

 first appearance of its cavity the heart surrounds the intes- 

 tine (figs. 41 and 67). Most specimens show the heart col- 

 lapsed with its walls in contact with the intestine, but some 

 specimens have it distended with blood. In all cases it is 

 easy to determine that the heart is perforated by the intes- 

 tine, but it is especially evident in specimens where the heart 

 is distended. In most of these cases the intestine lies nearer 

 the ventral than the dorsal wall of the heart, and in many 

 cases it lies directly in contact with this wall. At this stage 

 the heart is not separated into auricles and ventricle (fig. 



