Eyes of Molluscs and Arthropods. 709 



of energy. The sensitiveness of the organ depends upon 

 the niimber of scnse cells whieh have at that poiut been 

 converted into nerve fibres, or ganglionie cells, and may be 

 approxiinately estimated by the size of the group of gang- 

 liouic cells thus forrned by the modification of sensory ones. 



We have spoken of the cells modified for the reception of the eu- 

 ergy absorbed. We eonsider that dynamophagous organs are of two 

 natures; those formed by the modification of ciliated, and those forrned 

 by the modification of pigmented, or Chlorophyll cells. The former 

 receive energy in the shape of coarse movements of the surrounding 

 medium, or of fine vibrations like sound: and the latter, the still more 

 subtle vibrations of light. 



First, let US make a proposition, which we advance simply on the 

 gronnds of its plausibility and the difficulty we have found in explain- 

 ing certain facts in any other wa}', viz.: Animai pigment, 

 especially that of colorless plastids contained in the living 

 protoplasm of external, epithelial cells, is not a waste 

 product formed by the solar decomposition of protoplasm, 

 nor is it a protective covering, but a living substance with 

 a physiological activity necessary for the animai, and de- 

 pendent for this activity upon the presence of the sunlight. 



These pigment granules are modified Chlorophyll 

 granules, and like them have the power of absorbing solar 

 energy. 



There is little reason for supposing that pigment is a product of 

 decomposition. In plants, where it is well known that, under the in- 

 fluence of the sunlight, a Constant decomposition is in progress, there 

 are no pigmented waste products ; neither is there any color produced by 

 the decomposition of dead animai matter in the sun, while all other 

 well known waste products, such as carbonic acid gas, sulphides, am- 

 moniates, and ureates, are formed in abundance. The supposition that 

 pigment is a waste product of solar energy is not so different 

 from my own, as the supposition that pigment is simply a dead pro- 

 tective covering; for it is of little consequence, for the present, 

 whether we regard the pigment as a waste product of solar 

 energy. or as the receptive agent for the solar energy, so long 

 as we admit that the sunlight has an important beneficiai 

 effe et upon animai protoplasm. Most epithelial pigments, not 

 modified for special purposes, are very unstable, and therefore cannot 

 be regarded as waste products. In some cases they are extremely resis- 



