MOEPHOLOGY OF BRAIN AND SENSE ORGANS OF LIMULUS. 79 



dense white pigment. In Petromyzon the solubility of this 

 pigment in weak acids and its intensely white glistening 

 appearance in reflected^ and deep black appearance in trans- 

 mitted light has been the cause of contradictory statements 

 concerning it, some claiming it is black, others white^ and 

 others that it is absent, according to the method of prepara- 

 tion or the light (transmitted or reflected) by which it has 

 been examined. I have examined the white pigment in the 

 parietal eye of Limulus side by side with that in the parietal eye 

 of Petromyzon, and have found them to be identical in 

 appearance. In Limulus, as in Petromyzon, the white pig- 

 ment is much more soluble in dilute nitric acid than the black. 

 White pigment is comparatively rare in Invertebrates, and, so 

 far as I know, is confined to the Arthropods. It is present 

 about the base of the ommatidia in the compound eyes of the 

 crustaceans Pineeus and Galatea (Patten), and in Limulus is 

 found in the infolded margin of the young lateral eyes and 

 in the choroid plexus lying in front of the brain. But no- 

 where in Arthropods is it more abundant than in the endo- 

 parietal eye of Limulus. This eye is apparently nothing but 

 a solid mass of the pigment, and when ruptured with needles 

 it flows out in a dense white chalky stream. Ahlborn claims 

 that the white granules in Petromyzon are composed of calcium 

 phosphate, like the " brain-sand " found in the parietal eye of 

 the higher Vertebrates, It would be very interesting to learn 

 its composition in Limulus. 



Gaskell has compared the parietal eye of Vertebrates and 

 Crustaceans, but he failed to see the real points of resemblance 

 between them, for he knew nothing about their development 

 in Arthropods. There is no more resemblance between the 

 parietal eye of Vertebrates and the ocellus of Acilius, as he 

 puts it, than there is between the lateral eyes of Vertebrates 

 and those of a Cephalopod. If we fail to take into account 

 the development of the parietal eye in Arthropods there can 

 be no trustworthy grounds of comparison between it and the 

 parietal eye of Vertebrates. 



While there remains a good deal of doubt concerning the 



