562 CYCLOSTOMES 



Geotria (which genus some ichthyologists consider more primitive than, 

 perhaps ancestral to, the petromyzonids) are all cone-like in form, and 

 comprise three types in about equal numbers. The largest of these types 

 may however contain rhodopsin as does the short (rod) cell in the 

 petromyzonids — unlike the northern lampreys, Geotria is nocturnal, and 

 should have at least one type of functional rod. In any event, the average 

 petromyzonid pattern (Fig. 163b) shows neither an easy derivability 

 from that of Geotria (Fig. 163a) nor any ready convertibiUty into the 

 pattern of duplex selachians (Fig. 163c). 



No well-preserved material of Mordacia has ever been described. As 

 nearly as Franz could make out in his sections, most of the visual cells 

 are identical and are rod-like in form, with interesting 'false oil-droplets'. 

 The retina may truly be pure-rod, for the tiny pupil (0.2mm, in diam- 

 eter in a 3.0mm. eyeball) suggests a sensitive retina. In that case, it was 

 probably derived from an ancestral pure-cone one, something like that 

 which Geotria appears to have, by transmutation (see Plate I). 



(B) Hags 



In the hagfishes the eye may be nearly as large as that of a small 

 species of brook lamprey; but it is quite degenerate, and these animals 

 give no response to light. In Eptatretus and Polistotrema the eyeball, 

 1.0mm. (E. dombey) to 1.3mm. (P. stouti) in diameter, lies embedded 

 at the skinward side of a mass of fat three times its size, which in turn 

 is situated at a variable distance beneath the skin. There are no extra- 

 or intra-ocular muscles, no nerves except a vestigial optic, and there is 

 no pigment in either retina or uvea. There is no trace of a lens, though 

 in the embryo a lens placode forms and then thins out as if discouraged. 

 The sclera and chorioid are not differentiated from each other (c/. the 

 normal embryology of these tissue's — pp. 114-6); and the adult retina, 

 only half as thick (lOO[x) as the average vertebrate retina, is still actually 

 an optic cup with a considerable remnant of the old optic-vesicle cavity 

 (see Fig. 38, p. 106). In some individuals, the embryonic fissure persists. 



The eye of Myxine glutinosa is even more completely degenerate 

 (Fig. 133a, p. 387). The half-millimeter eyeball is practically filled by 

 the retina, which is doubled so sharply upon itself that there is no room 

 for a vitreous cavity. 



