An Experimental Study O F the Retina O F the Brook Lamprey , Entosphenus Appendix

The aim of the present investigation was to obtain a picture of the photomechanical changes in the retina of a cyclostome to lay alongside the results of a number of workers on the other vertebrate groups. The question as to whether the cyclostomes are to be considered a primitive group or as degenerate offshoots from another stock appears to be still open. The interest of investigators on both sides of the argument seems in recent years to have centered upon the eye more than on any one other anatomical feature. In a recent paper Keibel (’28) has gathered together the various arguments of the exponents of the ‘primitive’ theory, and upon the basis of his own observations upon the European brook lamprey, Petromyzon planeri, has disposed of them one by one and concluded that the eye of this form at least is specialized rather than primitive. Without going into the merits of the purely anatomical controversy, I wish to offer the results of this investigation as having a possible bearing on this question.