110 PARKER— THE RELATIONS OF THE 



central organ, it follows that this type of reaction must underlie that 

 of the more differentiated animals and must have been gradually 

 replaced by the kind of operation that we regard in ourselves as 

 spontaneous and volitional. That there is almost nothing in the re- 

 sponses of the higher animals that recalls a tropism is generally 

 ^ /moves away from the Hghty When sea-anemones are illuminated on 

 * one side, the pedal waves begin on that side and spread across the 

 foot to the opposite margin thus carrying the animal, without any 

 previous adjustment to the light, away from the source of illumina- 

 tion. When an earthworm is exposed to bright light it gradually 

 turns its anterior end away from the light and, thus directed, creeps 

 over a negative course. In all these tropic responses the animal falls 

 quickly into line through the influence of the stimulus on the general 

 receptive surface of its body and, without the necessary recourse to 

 specialized organs such as eyes, it takes a direction in relation to the 

 source of disturbance. There is not the least reason to suppose, 

 except possibly in the case of the earthworm, that these activities are 

 indicative of any sensational or other central-nervous element what- 

 ever. They are comparable with the movements of our internal 

 organs, such as the heart and the intestine, and from this view- 

 point they stand at an equally low and primitive level. They are in 

 every sense forced movements of the tropic variety. 



Probably much the same condition obtains in those animals that 

 are provided with the so-called eye spots. These are small photo- 

 receptors found on various places in different animals. They occur 

 on the edge of the bells of jellyfishes, at the ends of the arms of 

 starfishes and around the aboral pole of sea-urchins, on the heads of 

 many worms, of arthropod larvae, and perhaps of some snails. In 

 typical conditions they consist of a group of receptive cells sunk in 

 an open cup of pigment, so that the receptors are accessible to light 

 only from a generally restricted region, the light from the rest of the 

 field being received by the outer walls of the pigment cup. Such eye 

 spots are unprovided with devices for forming images, either pupils 

 or lenses. Occasionally lenses are present, but when such is the case, 

 the lens is concerned with the concentration of light and not with the 

 formation of an image. Such organs have been appropriately called 



