PARIETAL EYE OF THE PETROMYZON. 



143 



The inner and outer sacs of Petromyzon and the two similar ones in teleosts 

 have been regarded as right and left mates of a single pair, on the ground that 

 they are, for a short time at least, somewhat asymmetrical in position, one being 

 a little to the left, the other to the right of the median line; furthermore, it is claimed 

 that in the lamprey the larger outer sac is innervated mainly from the right 

 ganglion habenulae, and the inner one from the smaller, left ganglion habenulae. 



The evidence however, is ; by no means conclusive. My own observations 

 lead me to the conclusion that the inner and outer parietal eyes are just what they 

 appear to be, namely, two unpaired organs of slightly unequal value, one of which 

 has been crowded away from the median line. 



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FIG. 102. Semi-diagrammatic, sagittal sections of the head, showing the relative position and character 

 of the parietal eye, the epiphysis, neuropore, etc., in A, Branchipus; B, Apus; C, scorpion; D, Limulus; E, vertebrate. 

 On the right the eyes are shown on a larger scale. 



Evidence for this conclusion is afforded by the parietal eye of the cyclostomes 

 and less directly by the parietal eye of arachnids. 



i. In the first place, in the lampreys, during the earliest stages, one sac 

 lies directly behind the other, and there is nothing to indicate that one is the right 

 or left mate of the other. Whatever asymmetry appears in the eye sacs is seen 

 later, and is comparatively slight. The same condition appears to prevail in 

 teleosts, according to Hill's observations, although he interprets them differently. 



