It will be convenient, in describing the structural features of the group, to take as a basis for the description the marine lamprey, Petromyzon marinus.

This fact he connected with his observation that the fish Petromyzon, which possesses only two canals, moves in only two spatial dimensions.

In Petromyzon the kidney has the ordinary compact form, and here also the Malpighian bodies are shut off from the splanchnocoele.