THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 6 



O 



Eurypteridce— in fact, in the Palaaostraca generally — there are two 

 median eyes in addition to the lateral eyes, which were innervated 

 from these ganglia. 



In Ammoccetes, then, if the supra-infundibular portion of the 

 brain really corresponds to the supra-cesophageal of the paleeostracan 

 group, we ought to find, as indeed is the case, an optic apparatus 

 consisting of two lateral eyes and two median eyes, innervated from 

 the supra-infundibular brain-mass, and an olfactory apparatus built 

 up on the same lines as in the scorpion-group, also innervated from 

 this region. If, in addition, it be found that those two median eyes 

 are degenerate eyes of the same type as the median eyes of Limulus 

 and the scorpion-group, then the evidence is so strong as to amount 

 to a proof of the correctness of the theory. This evidence is precisely 

 what has been obtained in recent years, for the vertebrate did possess 

 two median eyes in addition to the two lateral ones, and these two 

 median eyes are degenerate eyes of the type found in the median 

 eyes of arthropods and are not of the vertebrate type. Moreover, as 

 ought also to be the case, they are most evident, and one of the 

 pair is most nearly functional in the lowest perfect vertebrate, 

 Anmiocoetes. 



Of all the discoveries made in recent years, the discovery that 

 the pineal gland of the vertebrate brain was originally a pair of 

 median eyes is by far the most important clue to the ancestry of 

 the vertebrate, for not only do they correspond exactly in position 

 with the median eyes of the invertebrates, but, being already 

 degenerate and functionless in the lowest vertebrate, they must have 

 been functional in a pre-vertebrate stage, thus giving the most direct 

 clue possible to the nature of the pre-vertebrate stage. It is 

 especially significant that in Limulus they are already partially 

 degenerated. What, then, ought to.be the structure and relation to 

 the brain of the median and lateral eyes of the vertebrate if they 

 originated from the corresponding organs of some one or other member 

 of the paheostracan group ? 



This question will form the subject of the next chapter. 



Summary. 



The object of this book is to attempt to find out from what group of inverte- 

 brates the vertebrate arose ; no attempt is made to speculate upon the causes of 

 variation by means of which evolution takes place. 



