Arrowheads have special meaning in ER diagrams. If you open an ERD template and select a line, you can click on Set Arrowheads in the Line Options control palette of the SmartPanel.
You can select the style of your arrow on both ends of your line to indicate cardinality and originality. Select the appropriate style of arrow by clicking on it for each end. The preview on the bottom will show what your line will look like.