THE SIMPLE EYE 



133 



These cells may belong to either of the two main types we have just 

 discussed. In the first place, we have already seen in the case of the 

 earthworm that an aggregation of apolar cells with a central organelle 

 in the cell-body may migrate from the surface epithelium to form a 

 subepithelial mass in association with the nerve fibres (Fig. 88). 

 Subepithelial eyes formed by the clumping together of a multitude of 

 visual cells of this tjrpe within a dense pigmentary mantle are found in 

 certain leeches. 



Figs. 89 and 



90. — The Sensory Organs of the Medicinal Leech, 

 HlRCDO Medici.\alis. 



Fig. 89. — The sensory organ of the inter- 

 mediate'segments consists of a collection 

 of undifferentiated sensory cells, S, 

 among which are seen the large light- 

 sensitive cells, V (4 in the figure), 

 with the kidney-shaped hyaline optic 

 organelle (after Biitschli). 



jg^trasOTiamjxr 



FiG. 90. — Each " eye "' situated 

 in the anterior segments con- 

 sists of a cluster of apolar 

 cells provided with optic 

 organelles, the whole being 

 enclosed in a pigment mantle 

 through which the nerve fibres 

 travel, and Ij'ing beneath the 

 surface epithelium (schematic 

 after Hesse). 



The ocelli of the medicinal leech, Hirudo medi'^inalis, are of unusual interest 

 since they show all stages of evolution from a unicellular to a multicellular eye. 

 As we have noted, ^ typical apolar light-sensitive cells may occur lying singly, 

 deep in the epithelium. On the dor.sal surface of the intermediate segments of 

 the animal there are paired clusters of undifferentiated sensory cells derived 

 from the epithelium, each cluster forming a segmental sensory organ the function 

 of which seems to be essentially tactile ; among these cells there are several 

 typical light-sensitive cells so that the colony presumably has a dual function 

 (Fig. 89). On the anterior five segments these clusters of cells are purely visual 

 and are clumped together in a cylindrical mass at right angles to the surface 

 enclosed in a dense pigmentary mantle, forming subepithelial eyes (Fig. 90) 



1 p. 131. 



Hirudo 



