ON THE ORIGIN OF VERTEBRATES. 229 



Comparison of the Vertebrate Median Eyes with the Median 



Arthropod Eyes. 



First I will consider the median or pineal eyes. I need not 

 treat them at any length, as I have already ^ fully stated why I 

 consider them to represent arthropod eyes of the simple type 

 of retina. They are exactly what one would expect to find as 

 the fading remnant of the " Hauptaugen." As I have already 

 remarked, their discovery is the most significant signpost of the 

 ancestry of the Vertebrate ; the structure of the best preserved 

 of the two eyes — the right pineal eye — in Ammocoetes resembles, 

 as I have shown in my paper, that of a median arthropod eye, 

 with its upright and simple retina. Since my first paper, 

 Leydig "' has written a long paper on the pineal eye, in which 

 he also comes to the conclusion that it was probably a median 

 eye of the arthropod type. 



Not only is the eye built up on the type of a median 

 arthropod eye, but its connection with the central nervous 

 system is of precisely the character distinguishing such eyes, 

 for its nerve is a simple nerve, arising from an optic ganglion 

 in exactly the same manner as is so characteristic of all eyes 

 with a simple retina. Nothing can be more convincing than 

 the appearance of the pineal nerve in my specimens, as seen in 

 my paper, unless it is the right so-called ganglion habenulse 

 into which that nerve enters, for this ganglion presents the most 

 typical appearance of such an optic ganglion with its cortical 

 layer of small granule cells arranged in rows, and its internal 

 medulla of fine reticulated substance (Punktsubstanz), just as is 

 seen in any picture of an optic ganglion of an arthropod eye. 



I repeat, then, that there is no difficulty with respect to the 

 pineal eyes. They (or, at all events, the most perfect one of 

 the pair) are absolutely comparable both in structure and 

 position to the two anterior median eyes of the spiders, and so 

 with the median eyes of the Arthropoda generally, for, seeing 

 the antiquity and universality of such eyes in the group, there 

 is every reason to consider the median eyes throughout as 

 homologous. 



1 Q. J. Micr. Scl., 1890. 



'•' Das Parietal organ d. Amphih. n. RcptiJ., Frankfurt a. M., 1890, 



