348 PHYSIOLOGY OF THE INVERTEBRATA. 



exactly the same parts as the eye of a Metazoon, with one 

 exception, the absence of the nerve-element." 



The ocular spots of other FlagcUata had been previously 

 investigated by Kiinstler, Claparede, and Lachman, and they 

 found crystalline humours and pigmented capsules (a choroid); 

 but what their true function was, they did not know, as no 

 nerve-apparatus fitted to perceive the impressions received 

 was, in the least, demonstrable in these organisms. On the 

 other hand, certain French savants state that "the co- 

 existence of a pigment and of a crystalline humour amply 

 suffices to characterize a visual organ. As to the nerve- 

 apparatus susceptible of perceiving impressions, it is re- 

 IDlaced by the protoplasm, which, as is well known, is sensi- 

 tive to light." It has also been stated, b}^ some observers, 

 that the red pigment in the ocular spots of the Protozoa 

 exhibits similar reactions to the pigment, which is present in 

 the rods of the retina of the Vcrfchrata. But it should be 

 borne in mind that pigment is not indispensable for the 

 sensation of light, because there are many eyes of complicated 

 structure from w^hich pigment may be altogether absent. 

 Therefore the only reasons which those observers, who state 

 that a visual organ is present in certain Protozoa, have for 

 such an assertion is, that the ocular spot has a definite 

 position, and it possesses a crystalline humour. Are these 

 facts sufficient to speak of it as an eye ? The Rev. W. H. 

 ])allinger, F.R.S., and Dr. J. Drysdale,* who examined the 

 ocular spots in various Monads, failed to discover the function 

 of these bodies after a most searching inquiry. 



In concluding this account of the sense-organs in the 

 Protozoa, it may be stated that the vesicles of Mliller in 

 LoModcs rof^frum (one of the Clliata) have been considered as 

 possessing an auditory function. 



* Tlte Monthly ^licroscoiical /anrnal, vol. II, p. 8 



