VEETEBEATES FEOM A CEUSTACBAN-LIKE ANCESTOE. 429 



In either case, whether Patten's or Lankester and Bourne's 

 description be accepted as the more correct, the type of 

 eye is clearly that of an Arthropod, and indeed of an ancient 

 form, for the arrangement of the nerve-end cells, the shape of the 

 internal cavity, the position and simplicity of the rhabdites, all 

 point to larval characteristics, and therefore to an ancient type. 



So far no mention has been made of any lens. One of the 

 characteristics of the Arthropod eye is the cuticular lens. In 

 the pineal eye of the Ammocoetes no observer has been able up 

 to this time to clearly point to any lens-like structure in that 

 eye. I entirely agree with Beard that the anterior wall of the 

 eye does not represent a lens. In my opinion, as just mentioned, 

 it does represent the corneagen. Where, then, is the lens ? 



4. The Cuticular Lens. — Upon the supposition that 

 we are dealing here with an eye of an Arthropodan type, it 

 follows that the lens must have been cuticular in structure, 

 and simply a local modification of the general cuticular covering 

 of the front part of the body. 



In all the horizontal sections through the eye it is very 

 plainly visible that the anterior wall of the eye is closely 

 pressed against the tissue which forms the wall of the cranial 

 cavity at this spot ; and in many cases it is very striking to see 

 how (as is represented in fig. 26, PI. XXVIII) the anterior wall 

 of the brain-case in this one place dips inwards, and is thickened 

 so as to form a projecting knob which is closely attached to the 

 pineal eye. The closeness of this attachment is seen when the 

 brain and eye are dissected out under a dissecting lens. I have 

 prepared many specimens in this way, and have found that with 

 care it is easy to cut away and remove the brain- case with the 

 overlying skin, and yet leave the eye in position and in con- 

 nection with the brain. In every case the eye then appears as 

 a white opaque round mass, in the centre of which anteriorly 

 the surface is not flat, but is hollowed out to form a distinct 

 manifest cup. The hollow of this cup was filled with the 

 projecting knob of the cranial wall, which was therefore removed 

 with the rest of that wall, and indeed can in some cases be 

 plainly seen under a dissecting lens as a slight projection on 



VOL. XXXI, PART III. NEW SEE. F F 



