78 WILLIAM PATTEN. 



nerve. The primary parietal eye-tube of Limulus corresponds 

 to the epiphysial outgrowth from the brain-roof iu Vertebrates. 

 The ectoparietal eyes containing the two separate retinas, and 

 the ventral diverticulum or endoparietal eye filled with white 

 pigment, correspond to the two distinct terminal organs found 

 in some Vertebrates. I venture to suggest that in Hatteria 

 the '^ capsular-like structure/' described by Spencer, at the base 

 of the eye where the nerve enters, corresponds to the endo- 

 parietal eye of Limulus. Again, the extraordinary presence of 

 dense pigment in the middle of the outer wall of the eye in 

 Varanus giganteus, as described by the same author, may be 

 regarded as the remnant of the pigmented epithelium origi- 

 nally separating the paired retinas of the ectoparietal eyes of 

 Limulus and scorpions ; and finally it is possible that the two 

 separate pineal eyes that occur in certain reptiles, such as 

 Anguis and Lacerta, may be due to shortening of the primary 

 evagination, so that the ecto- and endo-parietal eyes arise 

 as two independent outgrowths from the roof of the thalamen- 

 cephalon. 



The distal end of the primitive eye-tube in both Limulus 

 and Vertebrates is converted bodily into the pineal eye-stalk. 

 The proximal end in Limulus, after giving rise to the nerve- 

 roots, persists as an inverted T-tube ; the upright arm unques- 

 tionably corresponds to the epiphysis in Vertebrates, and the 

 cross-bar, perhaps, to the ganglion habenulse. The nerve- 

 roots correspond to the '' peduncles " of the pineal eye as seen 

 in mammals. In both Limulus and Vertebrates a large blood- 

 vessel accompanies the nerve to the pineal eye. I formerly 

 regarded the epiphysis as one of the nerves to the parietal eyes, 

 but it now seems to me to be nothing but the ectoderm along 

 which the parietal nerves formerly extended. The nerves 

 separate from it just as the peripheral nerves separate 

 from the overlying surface ectoderm. It may be compared to 

 the epithelium lining the interior of the hollow lateral eye- 

 nerves. 



A remarkable feature of the parietal eyes of Limulus and 

 of Vertebrates is the presence in them of great quantities of 



