242 



its broadest diameter. It lies closely applied to the renal vesicle along 

 its ventral edge, and in adult specimens lacks on that side not only 

 the pericardium, but the internal muscular layer as well. The diameter 

 of the lumen tapers off gradually at each end of the heart, but at a 

 point near which the wave of contraction begins, the lumen widens 

 somewhat giving a saccular appearance to each end of the heart. (See 

 Fig. 1.) 



«.Ä.^.^ 



Fia. J 



Fig. 1. Diagrammatic view of the heart and suiTounding tissues in Molgula man- 

 hattensis. X 25. a. h. g. ganglion at anterior (endoslylar) end of heart, p. h. g. gang- 

 lion at ijosterior end of heart, h. heart, o. ovaiy. r. v. renal vesicle, p. c. pericardial 

 chamber. 



Investigation of heart structure and action was made on living 

 and preserved material. Intravitam staining with methylen blue yeilded 

 good results. Meyer's B. X, methylen blue in l^/o solution in normal 

 salt was added to dishes containing living animals in a little water. 

 After remaining here for from ^2 to 3 hours the heart, with the sur- 

 rounding tissues, was removed in normal salt solution to a slide and 

 examined, first under a low power and then under a V12 Leitz immer- 

 sion. Of the preserved material Flemming, Hermann, and Vom Rath 

 osmic mixtures yielded the best results after staining with iron haema- 

 toxylin. 



The heart proper of Molgula is made up of three different ele- 

 ments, so far as the selective agency of methylen blue shows: first, 

 cross -striated muscle cells which do not take the stain; second, con- 

 nective tissue elements which are closely applied to the heart muscu- 

 lature in a somewhat regular manner; and third, nerve cells and fibers. 

 In the preserved material an endothelial lining can be made out. This 

 seems to be an extremely delicate pavement epithelium. When the 

 animal is in a normal position the heart adhers to and partly covers 



