142 THE PINEAL ORGAN 



the eyes of Eylais extendens, in which four eyes close together near the 

 median plane are enclosed in a dense capsule shaped like a spectacle 

 frame ; and the genus Hydroma in which two eyes enclosed in a chitinous 

 capsule are separated by the whole width of the dorsal shield, in the 

 centre of which is placed the organ which has been described as the 5th 

 median unpaired eye (Fig. 92). 



B. Hanstrom in 1926 published a full account of the genesis of the 

 eyes and visual centres of the Turbellaria, annelids, and arthropods, 

 and included in the latter some important observations on the eyes of 



f.S.v-^ 



Fig. 104. 



A — Dalmanites socialis 1/1. Dorsal aspect of cephalon, or head-shield, showing 

 the glabella with single median impression ; three primary dorsal furrows 

 and an occipital sulcus, which subdivide the glabella into five segments. 

 There is a V-shaped fixed cheek and a triangular free cheek bearing a pro- 

 minent facet eye ; the genal angles are prolonged backwards as strong, 

 tapering spines. 



B — Ordovician — Bohemia. Diagrammatic representation of similar specimen. 



/./. : frontal lobe. p. I. : palpebral fold. 



trilobites, Xiphosura, and Euryptera, in addition to those on living 

 representatives of the class. Commenting on the work of Barrande in 

 1872, he alludes to his illustration of the head of /Eglina prisca (Fig. 97, A), 

 showing three impressions on the glabella, which were regarded by 

 Handlirsch (1905) as indications of the existence in it of three highly 

 developed median eyes, corresponding to the three frontal eyes of insects. 

 This interpretation of the three impressions on the glabella of JEglina 

 is in Hanstrom 's opinion correct. For although the eyes of the trilobites 

 have the closest connections with the simple nauplius eyes of the crusta- 

 ceans, it would not be surprising if the median eyes of /Eglina, which 

 was distinctly a pelagic species, should have developed, as an adaptation 

 to a pelagic mode of life, and for the completion of the dorsal field of 



