5. The red blood-corpuscles are nucleated, biconvex, and oval.
6. The heart is (a) divided into two atria and one ventricle, and (b) it has a conus provided with valves.
7. The aortic arches are strictly symmetrical.
8. Gills are present at least during some early stages of development.
9. The kidneys are provided with persistent nephrostomes.
10. Lateral sense-organs are present at least during the larval stage.
11. The vagus is the last cranial nerve.
12. The median fins, where present, are not supported by spinal skeletal rays.
13. Sternal ribs and a costal or true sternum are absent.
14. There is no paired or unpaired medio-ventral, copulatory apparatus.