348 ME. H. M. BEENARD ON THE 



of their nerves and the masses of pigment enveloping the retinal cells, they seem, at 

 least in some cnses, to be functional. I have not been able to make out any more of 

 the finer details of these eyes than those which are given in the paper referred to. 



The great diversity in number, arrangement, and structure of the lateral eyes of Arachnids lends 

 some support to the view that they are secondary developments. A review of their positions and 

 numbers throughout the Arachnida inclines one to the belief that we have in the Aviculariidse the most 

 primitive condition. There we find an ocular tubercle with two large round median eyes, one on 

 each side of the median line. These median eyes are of the typical appearance of the median eyes 

 of the Arachnida, which generally seem to have a blue-greenish colour. On each side of the median 

 line, symmetrically an-auged, on the base of the tubercle, are three eyes differing in size, shape, and 

 appearance, as well from the median eyes as from one another. These are generally red or yellow, and 

 often very lustrous. Whether these accessory eyes of the tubercle are later developments than the 

 median eyes or not it is impossible to say, but I am inclined to think that, whenever they arose, they 

 belong in their place of origin to the ocular tubercle. 



In no other Arachnid are they now found on the tubercle, having, it appears, wandered off laterally 

 on to the cephalic lobes. 



In the Spiders they have not wandered far from the principal eyes, but are grouped in various ways 

 in front of and at tiie sides of the principal eyes, which, as a rule, retain their positions close to the 

 median line. This arrangement might clearly have been brought about by tlie obliteration of the optic 

 tubercle. 



In Phrynus the little groups of three lateral eyes on each side appear to have wandered ofiF from the 

 tubercle a short way outward and backward, leaving the median eyes in the original position, i. e. as 

 in Galeodes, on a tubercle anteriorly in the suture between the cej^halic lobes. 



In Thelyphonus the optic tubercle is obscured, but the median eyes retain their position upon the 

 somewhat modified anterior edge of the cephalothorax. The brilliant lateral eyes are in groups (three 

 in each in my specimens, as in Phrynus), but each group has travelled further back than in Phrynus, 

 and is at the edge of the dorsal surface, and far behind the median eyes. 



In Scorpio the median eyes frequently occur on an ocular tubercle, arranged at very different points 

 along the suture between the cephalic lobes. The lateral eyes appear to have wandered off when the 

 tubercle was in its original (anterior) position, inasmuch as, while the median eyes have wandered 

 backward, the lateral eyes are arranged near the anterior corners of the dorsal surface. They are in 

 groups of from five to two. 



The Phalangidae have retained the ocular tubercle, which is very prominent, owing to the tilting of 

 the eyes so as to look out sideways from the median line. As in many Scorpionidae, it has shifted back 

 along the cephalic suture. Lateral eyes occur in rare cases [Cyphophthalmus) , but it is a question 

 whether, in these cases, the lateral eyes are not the median eyes wandered apart. 



In the Chernetidse the median eyes have degenerated. The lateral eyes, arranged singly or in pairs, 

 are alone present at the lateral edges of the dorsal surface. It is, perhaps, possible that the lateral eyes 

 here, as in Cyphophthalmus, are the homologues of the median eyes, and that these median eyes 

 wandered apart, ou the obliteration of the ocular tubercle *. It is simpler, perhaps, to believe that the 

 eyes spread out from the tubercle, those in the middle line having degenerated. 



When the Acari have eyes they appear to be always laterally placed, and the same remarks apply 

 to them as to Chelifer. 



These lateral eyes are generally, if not in all cases, innervated from branches of the median optic 

 nerve, which supports the hypothesis that they wandered off the ocular tubercle. 



* MetBchnikoff (" Entwickelungsgeschichte des Chelifer," Z. w. Z. xxi. p. 71) makes no observations on the 

 origin ot the eyes. 



