no VERTEBRATES WITHOUT JAWS iv. 13- 



covered with cilia (Fig. 72), whose beat produces complicated 

 counter currents in the dorso-ventral plane. It has been suggested 

 that these function as a gyroscope, compensating for the absence of 

 a horizontal canal. 



In Myxine the condition is even simpler, there being only a single 

 vertical semicircular canal (Fig. 72). However, it is claimed that this 



Fig. 73. Horizontal section of the eye of a lamprey. 



er., ir., sr., external, internal, and superior rectus muscles; v.v. venous sinuses which cushion 

 the eye. (From Walls, The Vertebrate Eye, Cranbrook Institute of Science.) 



has cristae at both ends. The macular system also does not show the 

 characteristic subdivisions but is a single macula communis. 



14. Paired eyes of lampreys 



The structure of the paired eyes is similar to that in other verte- 

 brates. They are formed, like the pineal eyes, by evaginations of the 

 wall of the diencephalon; the so-called optic nerve is therefore not 

 really a peripheral nerve but a portion of the brain; it should strictly 

 be called the optic tract. The eyes are moved by extrinsic muscles 

 arranged in a somewhat unusual manner. Accommodation is effected 

 by a process found in no other vertebrates. The cornea consists of two 

 distinct layers, separated by a gelatinous substance. Attached to the 

 outer (or dermoid) cornea is a cornealis muscle, apparently of myo- 

 tomal origin, which flattens the cornea and pushes the lens closer 

 to the retina (Fig. 73). There is an iris, outlining a round pupil, which 

 changes little, if at all, in diameter under different illuminations. Most 

 species of lampreys are diurnal animals. They are said to move towards 



