THE EYES. 



355 



bony trabeculae belonging to the inner layer. There are no indications that 

 special organs, sensory or otherwise, were located in these openings. 



The Eyes. — The parietal and lateral eyes, whenever their position can be 

 certainly determined, are located, with the olfactory organs, on the median dorsal 

 portion of the cephalic buckler, forming in the antiarcha and aspidocephali a 

 very characteristic group. 



The lateral eyes in these families are remarkable. They are contained in 

 spherical chambers, the floor and sides of which are formed of a basket work of 

 bony trabeculae similar to those in Limulus. (Tremataspis, Cephalaspis Figs. 

 233, A, B, 239, A.) They are situated on short bony stalks that could be lowered 

 into the chamber, or the distal ends of the stalks could be raised, exposing the 

 convex surface of the oval or kidney-shaped cornea. (Fig. 239.) The latter is 

 covered by a thin shell that appears to be an extension of the dermal armor; 

 it is convex and smooth, and in life no doubt it was transparent (Tremataspis 

 and Cephalaspis). 



Fig. 233 , — A, Cross-section through the thoracic shield of Limulus, showing the location of the principal patches 

 of bony trabeculae; B, section through the head of Cephalaspis showing the bony trabeculee encasing the lateral 

 eyes, the thickened margin, and a part of the haemal wall of the shield; C, fringing processes of C. lyelli; D, same 

 of C. pagei; F, cross-section of the trunk of C. lyelli. 



In the pteraspidian and anaspidian sections, and in Ateleaspis, the lateral 

 eyes are apparently absent, or covered by a thick skeletal layer that, as in young 

 lampreys, effectively conceals their location. 



The apparent absence of the lateral eyes in certain ostracoderms is very 

 significant. There is no reason to suppose that the forms without lateral eyes 

 were cave animals, or deep sea animals, or that they belonged to an eyeless stock. 

 The unusually large size of the median eye tubercle, or fossa, and the well devel- 

 oped lateral eyes in their immediate relatives, is sufficient evidence to the contrary. 

 The temporary suppression of such ancient organs as the lateral eyes is best ex- 

 plained on the assumption that they are in a metamorphic, or transitional, condi- 

 tion, midway between the lateral eyes of arthropods, which remain outside the 



