OF MICRO-ORGANISMS. 23 



motor apparatus, do not exhibit any structure which 

 especially fits them for the reception of sensory im- 

 pressions. Similarly, Stein considers the vibratile 

 cilia as organs of touch. As these are organs which 

 have not undergone any differentiation, we shall not 

 stop to consider them. The Infusoria belonging to 

 the genus Cryptochilum (Maupas) carry at their pos- 

 terior extremity a long rigid bristle, which M. Maupas 

 regards as an organ of touch, intended to advise the 

 animal of the approach of other Infusoria. 



We shall speak more at length of the organ of 

 sight; this has been the subject of numerous treatises, 

 some of which are quite recent and of the greatest 

 interest to general physiology and psychology. Of 

 all the organs of sense the eye is the one which is 

 first differentiated. It is found in the organisms be- 

 longing to the vegetable kingdom as well as in those 

 belonging to the animal kingdom. While these small 

 beings do not seem to possess any organ especially 

 adapted by its structure for the reception of tactile, 

 olfactory, or gustatory impressions, a large number 

 already exhibit an ocular spot, that is to say a differ- 

 entiated organ, for the purpose of sight and for no 

 other purpose. 



Let us first turn our attention to the eye of the 

 Protozoa. 



It is chiefly in the group of Flagellates, and prin- 

 cipally in the species that are colored green by chlo- 

 rophyl (for example the Euglenae), that ocular spots 

 are found; these spots which are colored a bright red, 

 present themselves very clearly to the observation, 

 for they are set off by the uncolored plasma of 

 the anterior part of the body where they are generally 

 located. Oculiform spots are also found in the species 



