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J. B. JOHNSTON. 



into the ventral vessel very much as the valves in the parietal s 

 project into the dorsal. Thus, with the valves in the dorsal be- 

 tween each two pairs of hearts and the four valves in each heart, 

 regurgitation of blood during the strong cardiac contractions is 

 effectively guarded against. 



The study of the fine structure of the valves has presented 

 great difficulties because methods of fixation which give satisfac- 



Ti 3 .3. 



FIG. 3. A diagrammatic cross-section through one of the hearts to show the posi- 

 tion of the valves. The chloragogue and peritoneal epithelium are not drawn. 



tory preparations of all other tissues give very imperfect pictures 

 of these valves. This itself indicates one fact regarding their 

 structure, namely, that they are composed of very soft-bodied or 

 watery cells which may appear vacuolated or shrunken, or even 

 macerated. In many preparations the valves appear only as 

 masses of granular or coagulated material containing many ovoid 

 nuclei. In the most successful sections, however, the valves 



