TYPES OF VERTEBRATE AND INVERTEBRATE EYES 17 



eyes of certain crustaceans and the retina of the paired lateral eyes of 

 mammals (Figs. 11 and 12). Simple upright eyes are present in the lower 

 and some of the higher invertebrates and are also found in the median paired 

 eyes of both invertebrates and vertebrates (Figs. 9, 13, 14). They may 

 occur in the form of simple patches of modified epithelium, pits, or hollow 

 vesicles. From the inner ends of the epithelial cells elongated processes 

 or nerve-fibres are given off. These leave the deep surface of the plaque 

 or vesicle and join a subepithelial nerve-plexus or cerebral ganglion. 

 The sensory epithelial cells are often pigmented in one part, the rest of 



PRIMARY SENSE-CELLS NEUROSENSORY CELL SECONDARY SENSE-CELLS 

 * i * I , / 



ORCANOFCORTI 

 CENTRAL NERVOUS SYSTEM 



GANGLION CELLS 

 OF RETINA 



OLFACTORY CELLS 



GUSTATORY NERVE FIBRES 



Fig. 12. — Diagram illustrating the Supposed Mode of Growth of Primary 

 and Secondary Sensory Cells, and the Conversion of the Former into 

 Bipolar or Unipolar Neuro-sensory Cells. 



(Redrawn with some modifications from Arien-Kappers and Retzius.) 



the cell being clear, or specialized pigment-cells may lie between or 

 around the receptive cells. The outer end of a sensory or visual cell is 

 frequently rod-like, and is termed the rhabdite. This part of the cell is 

 clear and refractile, and it is believed to transmit the rays of light to the 

 body of the cell in which the nucleus is situated and which is continuous 

 at its inner end with a nerve fibre. In the more highly evolved types of 

 upright eye, a circular fold of epithelium enclosing a layer of mesenchyme 

 grows inward superficially and closing the mouth of an optic pit converts 

 it into an optic vesicle (Figs. 33-35, Chap. 3, pp. 48, 49). When in this way 

 the optic vesicle has been cut off, the epithelium lying over it and the under- 



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