Chapter 13 

 CYCLOSTOMES 



(A) Lampreys 



See also pages: 

 58 visual cells 

 117-8 embryology 

 126 Fig. 54c 



127, 131 signif. of larval lens 

 128 primitiveness of ependyma 

 135-6,210 Fig. 60, taxonomic position, 



habits, life-cycle 

 158 static pupil, lack of iris muscles, 



nocturnal migrations 

 177 outer nuclear layer 

 184, 187 lack of area centralis 

 191, 199 yellow coloration of lens 

 193-6 value of yellow lens 



251 Fig. 98 



259-60 accommodation 



264-5 optics 



268 function of vitreous 



272-3 Table VIII 



291 visual field 



338-9 median eyes 



371 intra-ocular fluids 



380 streamlining of eye 



390-1 parasitic habits 



406-7 macrophthalmia stage 



449-52 spertacle 



518-9 possibility of color vision 



537-8 dermal color changes 



Most lampreys live north of the equator, and these form the family 

 Petromyzonidse. In most of the genera of this family there are species 

 which are parasitic as adults. From each of these large lampreys, one 

 or more small, non-parasitic, 'brook' species has been derived. Some 

 slight simplification of the eye (but no true degeneration) has occurred 

 in all the brook lampreys, in keeping with the simplification of the whole 

 body and the life-cycle. 



South of the equator live two genera of parasitic forms, Geotria and 

 Mordacia, which differ somewhat from each other and from the pet- 

 romyzonid lampreys. Each perhaps deserves family rank; but their re- 

 lationships are not yet sufficiently well known. 



The Eye as a Whole — Of all non-degenerate vertebrate eyes, that of 

 the lamprey is the simplest. The ocular patterns of any two of the other 

 large groups of fishes will be found to differ from each other in only 

 one or two major characteristics. The lamprey eye lacks all of these 

 diagnostic features of higher fishes, and thus is primitive. But, it might 

 as easily add one feature as another : the lamprey eye is disappointingly 

 totipotential, and sheds no bright light upon the mode of origin of the 

 peculiarities of any 'higher' eyes. 



555 



