SENSORY REACTIONS 467 



while light was the initial stimulus for a few individuals, the 

 wild and excited swimming which gave the impression of great 

 sensitiveness to light was not due directly to this factor, but to 

 mechanical stimulation caused by mutual contact." 



Professor Parker found that Lancelets that had been kept in 

 the dark for some time would usually respond to light of not 

 more than a few candle-meters intensity, but that the same in- 

 dividuals after lengthy exposure to ordinary daylight often failed 

 to respond to a beam of strong sunshine. This is interesting 

 in showing that " the capacity of the animal to respond to light 

 is more or less determined by its previous condition, its sensitive- 

 ness diminishing with continual exposure to light and increasing 

 when the light is excluded from it ". Another interesting point 

 is that Amphioxus responds to a rapid ina'ease of light, but not 

 to a rapid decrease. It need hardly be said that by using a 

 heat-screen the experimenter made sure that the light-reactions 

 were not really reactions to radiant heat. 



The next step was to discover by what structures Lancelets 

 are sensitive to light. Is it by the whole skin, or by a con- 

 spicuous anterior pigment spot, or by a row of " eye-cups " which 

 lie along the nerve-cord ? Professor Parker devised an arrange- 

 ment for bringing a beam of light to bear on any desired part 

 of the animal's body. Experiment soon showed that the an- 

 terior pigment spot is not merely unimportant — but insensitive 

 — a result confirming what previous experimenters had noticed, 

 that the removal of the whole anterior tip made no difference 

 to the light-reactions. The exact portion of the Lancelet that 

 is stimulated by light corresponds to that in which the nerve- 

 tube contains the small " eye-cups" described by Hesse. " This 

 correspondence is so precise that it seems very probable that 

 these organs are the true photo-receptors." The skin may 

 have something to do with it, but the distribution of the sen- 

 sitiveness in different species corresponds to the distribution of 

 the " eye-cups ". 



Amphioxus swims away from light and comes to rest in 

 darkened situations. It cannot "see" in the sense of forming 

 an image, and " the light about it has little or no influence ex- 

 cept when it falls with full intensity on the animal's body". 

 This is to be connected with the fact that Amphioxus is 

 essentially a burrowing animal. Several observers have noted 



