TYPES OF VERTEBRATE AND INVERTEBRATE EYES 25 



confirmed by other workers in different species of Crustacea and insects. 

 The compound faceted eye was found to develop from a single invaginated 

 pit, a fact which proves that in these species the compound faceted eye is 

 not to be regarded as derived from ocelli which have coalesced during 

 ontogeny. Similar observations were made by Sedgwick, on Peripatus, 

 and Locy, on spiders. The details of this development will be considered 

 in the section on the eyes of crustaceans, and it will be only necessary to 

 mention here that the fusion of ocelli to form a single median eye usually 



Fig. 19. 



A — Freshwater Planarian. Pigment spots are situated on the dorsal aspect of 

 the head region, along the margin in front and laterally, and also two pairs 

 one behind the other on the top of the head near the middle line. 



B — Freshwater Planarian having a single pair of pigment spots on the dorsal 

 aspect of the head. 



C — A marine Turbellarian (Gimda) showing a single pair of pigment spots. 



D — Transverse section through the eyes of Dendroccelum lac tea. 



E — Section through one of the eyes of Dendrocozlum more highly magnified. 

 (R. J. G.) 



results in an eye of a simple type which shows regressive characters rather 

 than progressive evolution ; and the ontogeny of an organ which is highly 

 differentiated both in structure and function, such as the compound 

 faceted eyes, results from the subdivision of the individual cells of a simple 

 eye. This subdivision may take place (a) in a direction vertical to the 

 surface, producing (1) undifferentiated or slightly modified epidermal or 

 hypodermal cells, (2) retinal cells, and (3) ganglionic cells, continuous 

 with the cerebral ganglia ; and (b) transversely to the plane of the surface 

 epithelium, thus giving rise to groups of cells arranged round a single 



