82 Evolution and Distribution of Fishes 



As already noted, two of the typical four eyes present in 

 most species of Geonemertes and other related genera, 

 seem to have an exceptionally suggestive history, as tran- 

 sition is made from these to cyclostomes and higher verte- 

 brates, not least also, if accepted interpretations are cor- 

 rect, to some of the most ancient fishes. For from detailed 

 study of the past quarter century it is now generally ac- 

 cepted that the parietal eyes, or pineal and parapineal 

 organs, seen in lampreys and other vertebrates, represent 

 two more or less degenerate eyes. Studniclca's detailed 

 work on the European freshwater lamprey {47) and that 

 of Dendy on the New Zealand freshwater lamprey {48: 

 I ) as well as studies by the latter and by Cameron on 

 higher groups all fortify such a conclusion. In position 

 these arise behind the olfactory swelling, and in the lam- 

 prey a larger upper and a smaller lower one are in line 

 with each other. In the New Zealand lamprey — Geotria 

 — Dendy says "the larger and better developed of the two 

 does not lie above but behind the smaller and less well- 

 developed organ, so that both are distinctly visible when 

 the brain is viewed from above." He considers however 

 that they probably were paired organs primitively. 



As in the eyes of the higher nemerteans and of larval 

 lampreys, the pineal organ is circular and shows a retina, 

 a pigment layer, a pellucida, a capsule, and a ganglionic 

 cell-system, the last being prolonged from near the pos- 

 terior commissure. Dendy considered that the pineal organ 

 at least can still function as a light-perceptor. It is of 

 interest also to observe that in some large though primitive 

 and long extinct fishes to be described later, a distinct 

 opening or openings occur in the cephalic buckler, that is 

 probably indicative of the presence in these of parietal eyes. 



(c) The auditory organs. These have already been 

 generally referred to. Their internal structure has been 

 described by the writer (7:429-431) as follows: 



"The 'cerebral organs' of nemerteans have been much 

 discussed, though unanimity of opinion as to their function 

 has not yet been reached. These however seem, like most 

 other parts of nemerteans, to be the simple rudiments 

 of more evolved structures in vertebrates. They are 



