350 



THE PINEAL ORGAN 



or cornea through which the lenses of the ommatidia are visible by trans- 

 lucence — Asaphus, Illcenus, Calymene (Fig. 97, A) ; and (2) schizochroal, 

 in which the cornea is transected by protrusion of the sclera between the 

 ommatidia and is limited to or present only on the surfaces of the omma- 

 tidia (Fig. 97) — Phacops, Daimanites, ? Harpes. Barrande recognized a 

 third type which is exemplified in the genus Harpes, the eye of which is 

 regarded as an aggregate of ocelli which are disposed in groups of two or 

 three together, a circumstance which may be regarded as indicating a 

 primitive stage in the evolution of compound faceted eyes in which the 

 ommatidia are uniformly disposed, are close together and are covered 



o.afc 



c.ev 



Fig. 247. — Sagittal Section through the 



Apus. (After 



A. : anterior. 



a. rt. : anterior retina. 



c. ev. : common cavity which encloses 

 the lateral eye vesicles and into 

 which the cavity, at., of the parietal 

 eye vesicle previously opened, at 

 the recess marked o. at. 



m. : fold covering the lateral eyes. 



Parietal Eye Vesicle of an Adult 

 Patten.) 



: opening of common cavity. 



: posterior. 



rt. : posterior retina. Each placode 

 consists of a single row of large 

 colourless columnar cells the distal 

 ends of which are buried in a dense 

 mass of pigment, their proximal 

 ends being clear. 



by a continuous cornea. Harpes ungula (Fig. 97, B), in which this primitive 

 condition of the lateral eyes is met with, belongs to the Ordovician period, 

 the age of which is estimated to be 480,000,000 to 590,000,000 years ago. 

 The three impressions on the glabella of /Eglina prisca (Fig. 97, A), 

 which are considered by some eminent palaeontologists to represent 

 ocelli, are arranged in the form of a triangle with the apex forward. In 

 Daimanites socialis (Fig. 104, A) a single impression is present. In a similar 

 specimen of Daimanites three impressions are found, disposed in the 

 form of a triangle with the apex behind (Fig. 104, B), while in certain 

 examples of Trinucleidae there are five impressions on the surface of the 



